Add "generic" key methods for quickly deriving keys from
key component lists in a bijective manor. This is useful
for BagOStuff classes that wrap other BagOStuff instances
or for parsing keys to get stats.
Make the proxy BagOStuff classes (ReplicatedBagOStuff,
MultiWriteBagOStuff, CachedBagOStuff) use "generic" keys
so that they can convert to appropriate keys when making
backing cache instance method calls.
Make EmptyBagOStuff, HashBagOStuff, APCUBagOStuff,
RedisBagOStuff, and RESTBagOStuff use "generic" keys rather
than those of MediumSpecificBagOStuff::makeKeyInternal().
This lets proxy BagOStuff classes bypass key conversions
when used with instances of these classes as backing stores.
Also:
* Fix missing incr(), incrWithInit(), and decr() return
values in MultiWriteBagOStuff.
* Make MultiWriteBagOfStuff, ReplicatedBagOStuff, and
CachedBagOStuff use similar backend method forwarding
styles by using a new BagOStuff method.
* Improved various related bits of documentation.
Bug: T250239
Bug: T235705
Change-Id: I1eb897c2cea3f5b756dd1e3c457b7cbd817599f5
5c335f9d77