WIP rollup / delete with async workers
Rollup the counter entries into a PN-Counter in the background

When some threshold of increments is detected, have a pool worker
roll up those increments into the PN-Counter, and delete all
accounted for keys. Trying to keep the op log small so a read
doesn't need to fold over too many keys at a time.
Remove old worker impl
Merge pull request #1 from russelldb/background-rollup

Background rollup