libpulse: Store pa_stream pointers to hashmaps instead of dynarrays.

libpulse: Store pa_stream pointers to hashmaps instead of dynarrays.

Since the stream identifiers (channels) are monotonically growing integer, it
isn't a good idea to use them as index to a dynamic array, because the array
will grow all the time. This is not a problem with client connections that
don't create many streams, but, for example, long-running clients that use
libcanberra for playing event sounds, this means that the client connection
effectively leaks memory.