SiteSession: make searchTags() unsynchronized

synchronize this function make SearchView stall.
since we do our own lockings with ReadWriteLock, this should be fine.
GelbooruAPI: allow ssome tag property to fail

sometimes the server fails to give us the correct XML (?).
if it did, just ignore that tag.
PostList: just restartLoader()

instead of cancelLoad() -> startLoading().

according to the documentation, when restartLoader(), the ond loader is
destroyed and a new one is initiated.