pbx: Hold channel lock for exception datastore access

pbx: Hold channel lock for exception datastore access

ast_channel_datastore_find() and ast_channel_datastore_add() must only be
called while the channel is locked (see channel.h). raise_exception() and the
EXCEPTION dialplan function read path accessed the exception datastore without
holding ast_channel_lock, which could corrupt the per-channel datastore list
under concurrency and lead to crashes during teardown (e.g. double free in
ast_datastore_free).

Resolves: #1831