Fixes Post#get_count_from_cache to only lookup the count in the tags
table when the search is for a single "simple" tag.
Check memcache when the search is not a simple tag:
* multi-tag searches (touhou rating:s)
* single metatag searches (rating:s, source:"foo bar")
* negated tags (-touhou)
* wildcard tags (*touhou*)
* or searches (~touhou)