[PATCH 0 of 5] v2: setup refactoring for persistent tag caching

[Greg Ward]

Take 2.  Now I've got the design of _findtags() right early in the
patch series (thanks to djc for just the nudge I needed).  I knew that
was the right decision when one patch suddenly became unnecessary.
Woo hoo.  Leaving out the final patch that actually implements
persistent caching because, well, it's broken right now.

IMHO these 5 patches are done, but please don't push them yet: I want
to keep the patch series intact.  This is strictly for review.  So
please, have at it!

Greg