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/common.h

@@ -193,7 +193,6 @@ gchar** format_call_external_argv(const char* template, const char* url, const c
 gchar* unique_filename_from_url(const char* url, const char* path);
 gchar* get_expanded_path(const char* path);
 
-void glib_hash_table_free(GHashTable* hash_table);
 char* basename_from_url(const char* url);
 
 gchar* prof_get_version(void);