Revert "objectcache: fix race conditions in RedisBagOStuff::incr()"

Revert "objectcache: fix race conditions in RedisBagOStuff::incr()"

This commit reverts most of 7e647d2f0f5b, but keeps unrelated code
clean ups from it, as well as the conflicting changes from d8b952ae47.

From WMF production nutcracker:

> nc_redis.c parsed unsupported command 'WATCH'

The use of WATCH, in addition to failing the commands that use it,
also appears to also have caused a chain reaction making nutcracker
intermittently unavailable to other web requests.

Bug: T228303
Change-Id: Ic37efc2963b147e461837571ae0b65acf3f60cb4