Crash while compaction in background on iOS

using couchbase-lite-ios 1.4 (via pod install)

The following crash occurs rather frequently when database compaction starts while app is started in background (e.g. via background fetch or silent push message)
Compaction is started automatically, not manually.

```
Crashed: Thread
0 PurpleAnt 0x1004d2894 std::__1::__hash_const_iterator, std::__1::allocator >, bool>, void*>*> std::__1::__hash_table, std::__1::allocator >, bool>, std::__1::__unordered_map_hasher, std::__1::allocator >, std::__1::__hash_value_type, std::__1::allocator >, bool>, std::__1::hash, std::__1::allocator > >, true>, std::__1::__unordered_map_equal, std::__1::allocator >, std::__1::__hash_value_type, std::__1::allocator >, bool>, std::__1::equal_to, std::__1::allocator > >, true>, std::__1::allocator, std::__1::allocator >, bool> > >::find, std::__1::allocator > >(std::__1::basic_string, std::__1::allocator > const&) const (__hash_table:2228)
1 PurpleAnt 0x1004d133c cbforest::Database::getKeyStore(std::__1::basic_string, std::__1::allocator >) const (unordered_map:1121)
2 PurpleAnt 0x1004d19e8 cbforest::Database::updatePurgeCount() (Database.cc:227)
3 PurpleAnt 0x1004d10b4 cbforest::Database::compactionCallback(_fdb_file_handle*, unsigned int, char const*, fdb_doc_struct*, unsigned long long, unsigned long long, void*) (Database.cc:406)
4 PurpleAnt 0x100516d78 _fdb_compact_file(_fdb_kvs_handle*, filemgr*, btreeblk_handle*, docio_handle*, hbtrie*, hbtrie*, btree*, btree*, unsigned long long, bool) + 8200
5 PurpleAnt 0x1005154c4 fdb_compact_file + 1876
6 PurpleAnt 0x100501624 compactor_thread(void*) + 22196
7 libsystem_pthread.dylib 0x18a3e568c _pthread_body + 240
8 libsystem_pthread.dylib 0x18a3e559c _pthread_body + 282
9 libsystem_pthread.dylib 0x18a3e2cb4 thread_start + 4
```