Migrate meta to llmeta

[mponomar]

Signed-off-by: Michael Ponomarenko [email protected]

This adds code that adds llmeta keys for things that used to be placed in the old 'meta' dbs,. It also adds a couple of tunables for converting on startup, double writing to both stores, and reading from the new store. This is a path forward to getting rid of old non-singlemeta databases, and eventually getting rid of meta altogether.