perf: Contacts: Lookup by strings instead of JID objects
About 2x faster now:
@bq:17:23:14:/media/d/temp/git/gajim$ cat stats|grep 'contacts\.py'|grep get_contact
266640 3.774 0.000 8.969 0.000 contacts.py:126(get_contact)
@bq:17:23:17:/media/d/temp/git/gajim$ cat stats2|grep 'contacts\.py'|grep get_contact
222645 2.083 0.000 3.765 0.000 contacts.py:132(get_contact)