Memory leak: Investigate valgrind output after #1073

**Describe the bug**

Even after #1073, valgrind shows some possible leaks. These should be investigated.

Note: I'm not entirely sure if they are real or relevant leaks. I suppose there may be some ressources which are not destroyed explicitly when Jamlus shuts down. This would be not relevant in the real world, but would probably still be shown as a possible leak by valgrind.

So, I'm not saying that all of the outputs are worth fixing, I'm just proposing to investigate their validity and relevance.

**To Reproduce**

```
$ valgrind --leak-check=full ./Jamulus -s -n -p 10001 -e localhost:10000 -l /tmp/log -R /tmp/recordings -o 'Private Server;Stockholm;205' -m /tmp/jam.dat -u 40 -T -w 'Welcome!'

$ for x in {1..12}; do Jamulus -c localhost:10001 --nogui --inifile /tmp/x &; done

# leave running for some minutes
# Ctrl+C and wait for exit (takes a rather long time, possibly due to valgrind overhead + recording finishing?)
```

**Expected behavior**
No leaks.

**Screenshots**

```
==3121==
==3121== HEAP SUMMARY:
==3121== in use at exit: 8,560,244 bytes in 1,231 blocks
==3121== total heap usage: 11,163,830 allocs, 11,162,599 frees, 3,097,366,260 bytes allocated
==3121==
==3121== 352 bytes in 1 blocks are possibly lost in loss record 110 of 156
==3121== at 0x483CB65: calloc (vg_replace_malloc.c:760)
==3121== by 0x40142EB: _dl_allocate_tls (in /usr/lib/ld-2.32.so)
==3121== by 0x48C913C: pthread_create@@GLIBC_2.2.5 (in /usr/lib/libpthread-2.32.so)
==3121== by 0x58D79A2: QThread::start(QThread::Priority) (in /usr/lib/libQt5Core.so.5.15.2)
==3121== by 0x15C7D2: CServer::CServer(int, QString const&, unsigned short, QString const&, QString const&, QString const&, QString const&, QString const&, QString const&, QString const&, bool, bool, bool, bool, ELicenceType) (in /home/christian/Projects/jamulus/Jamulus)
==3121== by 0x13CD8F: main (in /home/christian/Projects/jamulus/Jamulus)
==3121==
==3121== 352 bytes in 1 blocks are possibly lost in loss record 111 of 156
==3121== at 0x483CB65: calloc (vg_replace_malloc.c:760)
==3121== by 0x40142EB: _dl_allocate_tls (in /usr/lib/ld-2.32.so)
==3121== by 0x48C913C: pthread_create@@GLIBC_2.2.5 (in /usr/lib/libpthread-2.32.so)
==3121== by 0x58D79A2: QThread::start(QThread::Priority) (in /usr/lib/libQt5Core.so.5.15.2)
==3121== by 0x156F4F: CServer::Start() (in /home/christian/Projects/jamulus/Jamulus)
==3121== by 0x5AEC4BE: QObject::event(QEvent*) (in /usr/lib/libQt5Core.so.5.15.2)
==3121== by 0x5ABFA4F: QCoreApplication::notifyInternal2(QObject*, QEvent*) (in /usr/lib/libQt5Core.so.5.15.2)
==3121== by 0x5AC2572: QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) (in /usr/lib/libQt5Core.so.5.15.2)
==3121== by 0x5B190A3: ??? (in /usr/lib/libQt5Core.so.5.15.2)
==3121== by 0x7147A83: g_main_context_dispatch (in /usr/lib/libglib-2.0.so.0.6600.4)
==3121== by 0x719B9B0: ??? (in /usr/lib/libglib-2.0.so.0.6600.4)
==3121== by 0x71462B0: g_main_context_iteration (in /usr/lib/libglib-2.0.so.0.6600.4)
==3121==
==3121== 1,408 bytes in 4 blocks are possibly lost in loss record 135 of 156
==3121== at 0x483CB65: calloc (vg_replace_malloc.c:760)
==3121== by 0x40142EB: _dl_allocate_tls (in /usr/lib/ld-2.32.so)
==3121== by 0x48C913C: pthread_create@@GLIBC_2.2.5 (in /usr/lib/libpthread-2.32.so)
==3121== by 0x58D79A2: QThread::start(QThread::Priority) (in /usr/lib/libQt5Core.so.5.15.2)
==3121== by 0x58DA754: QThreadPoolPrivate::startThread(QRunnable*) (in /usr/lib/libQt5Core.so.5.15.2)
==3121== by 0x58DBB13: QThreadPoolPrivate::tryStart(QRunnable*) (in /usr/lib/libQt5Core.so.5.15.2)
==3121== by 0x58DC327: QThreadPool::start(QRunnable*, int) (in /usr/lib/libQt5Core.so.5.15.2)
==3121== by 0x19276F: QFuture QtConcurrent::run(CServer*, void (CServer::*)(int, int, int), int const&, int const&, int const&) (in /home/christian/Projects/jamulus/Jamulus)
==3121== by 0x15A393: CServer::OnTimer() (in /home/christian/Projects/jamulus/Jamulus)
==3121== by 0x5AEC581: QObject::event(QEvent*) (in /usr/lib/libQt5Core.so.5.15.2)
==3121== by 0x5ABFA4F: QCoreApplication::notifyInternal2(QObject*, QEvent*) (in /usr/lib/libQt5Core.so.5.15.2)
==3121== by 0x5AC2572: QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) (in /usr/lib/libQt5Core.so.5.15.2)
==3121==
==3121== 1,824 (16 direct, 1,808 indirect) bytes in 1 blocks are definitely lost in loss record 138 of 156
==3121== at 0x483ADEF: operator new(unsigned long) (vg_replace_malloc.c:342)
==3121== by 0x155F3A: recorder::CJamController::SetRecordingDir(QString, int, bool) (in /home/christian/Projects/jamulus/Jamulus)
==3121== by 0x15B799: CServer::CServer(int, QString const&, unsigned short, QString const&, QString const&, QString const&, QString const&, QString const&, QString const&, QString const&, bool, bool, bool, bool, ELicenceType) (in /home/christian/Projects/jamulus/Jamulus)
==3121== by 0x13CD8F: main (in /home/christian/Projects/jamulus/Jamulus)
==3121==
==3121== 9,504 bytes in 27 blocks are possibly lost in loss record 153 of 156
==3121== at 0x483CB65: calloc (vg_replace_malloc.c:760)
==3121== by 0x40142EB: _dl_allocate_tls (in /usr/lib/ld-2.32.so)
==3121== by 0x48C913C: pthread_create@@GLIBC_2.2.5 (in /usr/lib/libpthread-2.32.so)
==3121== by 0x58D79A2: QThread::start(QThread::Priority) (in /usr/lib/libQt5Core.so.5.15.2)
==3121== by 0x58DA754: QThreadPoolPrivate::startThread(QRunnable*) (in /usr/lib/libQt5Core.so.5.15.2)
==3121== by 0x58DBB13: QThreadPoolPrivate::tryStart(QRunnable*) (in /usr/lib/libQt5Core.so.5.15.2)
==3121== by 0x58DC327: QThreadPool::start(QRunnable*, int) (in /usr/lib/libQt5Core.so.5.15.2)
==3121== by 0x19276F: QFuture QtConcurrent::run(CServer*, void (CServer::*)(int, int, int), int const&, int const&, int const&) (in /home/christian/Projects/jamulus/Jamulus)
==3121== by 0x15A015: CServer::OnTimer() (in /home/christian/Projects/jamulus/Jamulus)
==3121== by 0x5AEC581: QObject::event(QEvent*) (in /usr/lib/libQt5Core.so.5.15.2)
==3121== by 0x5ABFA4F: QCoreApplication::notifyInternal2(QObject*, QEvent*) (in /usr/lib/libQt5Core.so.5.15.2)
==3121== by 0x5AC2572: QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) (in /usr/lib/libQt5Core.so.5.15.2)
==3121==
==3121== LEAK SUMMARY:
==3121== definitely lost: 16 bytes in 1 blocks
==3121== indirectly lost: 1,808 bytes in 10 blocks
==3121== possibly lost: 11,616 bytes in 33 blocks
==3121== still reachable: 8,546,804 bytes in 1,187 blocks
==3121== suppressed: 0 bytes in 0 blocks
==3121== Reachable blocks (those to which a pointer was found) are not shown.
```

**Operating system**

```Linux wuechoo 5.10.9-arch1-1 #1 SMP PREEMPT Tue, 19 Jan 2021 22:06:06 +0000 x86_64 GNU/Linux
```

**Version of Jamulus**

3.6.2dev-a26ff711 (= #1073)

@softins Are you already planning to look further into this? I can try my luck, but you seem way more proficient in tracking such things down. :)

Reproduce the report with the provided Valgrind command and workload, then inspect the CServer::CServer, CServer::Start, CServer::OnTimer, and recorder::CJamController::SetRecordingDir traces. Compare the definitely lost and possibly lost records with the Qt thread and recording shutdown behavior; done means identifying which reports are real and relevant and addressing those that are valid.

Written by the indexing model from the issue text.