Use uuid in create_unique_id instead of counter

Message IDs should be unique so they can be used by XEPs like delivery receipts, chat markers, message correction.

So far it used a counter so restarting profanity will cause the counter
to be 0 again.

Let's rather use an UUID since we have such a function in the
xmpp/xmpp.h already.

Closes https://github.com/boothj5/profanity/issues/998

src/common.h

@@ -94,7 +94,6 @@ gboolean release_is_new(char *found_version);
 
 char* p_sha1_hash(char *str);
 char* create_unique_id(char *prefix);
-void reset_unique_id(void);
 
 char* get_file_or_linked(char *loc, char *basedir);
 char* strip_arg_quotes(const char *const input);