objectcache: make RedisBagOStuff pass all tests

* Provide a default lock-based BagOStuff::cas implementation
* Make RedisBagOStuff::merge() use mergeViaCas()
* Use the raw unserialized string as the redis CAS token to
  avoid any bad interaction with __wakeup() methods changing
  field values every time
* Make RedisBagOStuff::incr() return false when there is no
  such key, not null
* Rewrite merge() test to make the order of write/cas phase
  of the parent and child merge() calls well defined instead
  of arbitrary usleep() calls
* Avoid cache key reuse in test runs

Change-Id: I388ec173cf3858bb2fc7a8c8a00cda68703074ce