Add TSDB.assignUid() to assign UIDs to metric, tagks and tagvs using the caches.
It's also slightly different than UniqueId.getOrCreateId() in that we want to
throw an exception if the UID is already assigned.
Add unit test file for the TSDB class with tests for assignUID(), need more
Set Tags.validateString() public as we'll need it elsewhere

Signed-off-by: Chris Larsen <clarsen@euphoriaaudio.com>