Add caching for KeySerializers, similar to regular value serializers

[cowtowncoder]

(spin-off from #943)

Although it is possible to add local caching for key serializers of Map types, it may make sense to also add global cache, used via SerializerProvider.findKeySerializer(...). This would reduce cost of lookup misses from local cache, or eliminate need for local caches altogether.

The reason caching is important, in general, is that due to extensive configuration via annotations, modular extension mechanisms, optional contextualization and other advanced feature, the actual lookup by type is rather a complicated process. So instead of trying to go crazy optimizing lookups (which may or may not even be possible; any module can request callbacks to be done during lookups) it often makes sense to make sure resulting handler is cached for future (dynamic) lookups.

[cowtowncoder]

Still not sure it'll get in 2.8 so...

[cowtowncoder]

At this point question is this: while adding cache would be easy enough, is there data to show that 2 levels of cache (global for serializers, local for "dynamic keys") does not work?
If yes, can be worked on, solve. But right now I don't have anything; and none of test cases (including "wasteful ones") can quite show problem.

So, closing for now.