Rework OMEMO handling on disconnect

* Fix missing destruction of `session_store` and mutex
* Replace `glib_hash_table_free()`
  The glib API `g_hash_table_destroy()` does exactly the same.
* Use the default libsignal `destroy_func` instead of doing that manually
* Set internal state to `0` after everything is cleaned up

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>

src/omemo/omemo.h

@@ -51,8 +51,6 @@ typedef enum {
     PROF_OMEMOPOLICY_ALWAYS
 } prof_omemopolicy_t;
 
-typedef struct omemo_context_t omemo_context;
-
 typedef struct omemo_key
 {
     unsigned char* data;