[BUG] Memory leaks - Decoder in thread

[madkote]

Similar issue as with the model (#36).

Scenario

decoder n files in a list with

• option A - use same decoder

decoder = KaldiNNet3OnlineDecoder(model)
try:
    for filename in [...]:
        decode(decoder, filename)
finally:
    del decoder

• option B - create a new decoder for each file

def decode(filename):
  decoder = KaldiNNet3OnlineDecoder(model)
  try:
     ...
  finally:
    del decoder

for filename in [...]:
    decode(filename)

• option C - when running option A and B in separate thread, the memory usage increases by ~10mb for each file, even del decoder is used. Deleting the model has not effect, since it leaving much bigger leak in the memory (see [BUG] Memory leaks - Model #36)