Proper cleanup on gpu server interrupt

```
Traceback (most recent call last):
File "./asr_server_gpu.py", line 92, in
start()
File "./asr_server_gpu.py", line 88, in start
loop.run_forever()
File "/usr/lib/python3.7/asyncio/base_events.py", line 539, in run_forever
self._run_once()
File "/usr/lib/python3.7/asyncio/base_events.py", line 1739, in _run_once
event_list = self._selector.select(timeout)
File "/usr/lib/python3.7/selectors.py", line 468, in select
fd_event_list = self._selector.poll(timeout, max_ev)
KeyboardInterrupt
ASSERTION_FAILED ([5.5.1008-d81a9]:~BatchedThreadedNnet3CudaOnlinePipeline():batched-threaded-nnet3-cuda-online-pipeline.cc:51) Assertion failed: (available_channels_.empty() || available_channels_.size() == num_channels_)

[ Stack-Trace: ]
/home/shmyrev/gpu-build/vosk-api/python/gpu-env/lib/python3.7/site-packages/vosk-0.3.32-py3.7.egg/vosk/libvosk.so(kaldi::MessageLogger::LogMessage() const+0x766) [0x7f0526dfb896]
/home/shmyrev/gpu-build/vosk-api/python/gpu-env/lib/python3.7/site-packages/vosk-0.3.32-py3.7.egg/vosk/libvosk.so(kaldi::KaldiAssertFailure_(char const*, char const*, int, char const*)+0x5b) [0x7f0526dfc196]
/home/shmyrev/gpu-build/vosk-api/python/gpu-env/lib/python3.7/site-packages/vosk-0.3.32-py3.7.egg/vosk/libvosk.so(kaldi::cuda_decoder::BatchedThreadedNnet3CudaOnlinePipeline::~BatchedThreadedNnet3CudaOnlinePipeline()+0x669) [0x7f0526a422bd]
/home/shmyrev/gpu-build/vosk-api/python/gpu-env/lib/python3.7/site-packages/vosk-0.3.32-py3.7.egg/vosk/libvosk.so(BatchRecognizer::~BatchRecognizer()+0x14c) [0x7f0526a37c6c]
/home/shmyrev/gpu-build/vosk-api/python/gpu-env/lib/python3.7/site-packages/vosk-0.3.32-py3.7.egg/vosk/libvosk.so(vosk_batch_recognizer_free+0xe) [0x7f0526a375de]
/lib/x86_64-linux-gnu/libffi.so.6(ffi_call_unix64+0x4c) [0x7f0528d0f8ee]
/lib/x86_64-linux-gnu/libffi.so.6(ffi_call+0x22f) [0x7f0528d0f2bf]
/home/shmyrev/gpu-build/vosk-api/python/gpu-env/lib/python3.7/site-packages/_cffi_backend.cpython-37m-x86_64-linux-gnu.so(+0x19f97) [0x7f0528d2cf97]
python3(_PyObject_FastCallKeywords+0x4eb) [0x5ce4ab]
python3() [0x53ed81]
python3(_PyEval_EvalFrameDefault+0x49ba) [0x54653a]
python3(_PyFunction_FastCallDict+0x186) [0x5ce9c6]
python3() [0x5830aa]
python3() [0x518d23]
python3(_PyGC_CollectNoFail+0x2b) [0x6280db]
python3(PyImport_Cleanup+0x458) [0x52b918]
python3(Py_FinalizeEx+0x6e) [0x6314de]
python3() [0x6526cd]
python3(_Py_UnixMain+0x2e) [0x65290e]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xeb) [0x7f052992c09b]
python3(_start+0x2a) [0x5d531a]

```