Move all decoder instances and callbacks into a session struct

The struct srd_session also holds configuration parameters formerly
passed to srd_session_start().

The new call srd_session_new() takes a pointer where a newly allocated
pointer to struct srd_session will be stored. This pointer must be
passed as the first argument to all functions that require it.

The SRD_CONF_NUM_PROBES, SRD_CONF_UNITSIZE and SRD_CONF_SAMPLERATE
keys must be configured with srd_config_set() before srd_session_start()
is called.

A new call srd_session_destroy() is also available. This cleans up all
resources the session holds. This is also called from srd_exit() for
all sessions.