DecoderStack: Make decoder sessions terminate after running

Currently, SRD sessions aren't terminating after all data
has been decoded, leaving the decode thread in a state
where it's waiting for more data.
This would be okay if an acquisition was still ongoing but
when the acquisition has been stopped, there will never be
more data. When a new acquisition is started, the previous
decode sessions will be terminated and then new ones will
be started. This is the reason why this behavior wasn't
an issue up until now.

However, the fix for #181 requires that the decode thread
terminates or else the global SRD lock will never be
released.