Limit size of `SerializerCache`, auto-flush on exceeding

[cowtowncoder]

(note: offshoot of #1905)

Currently size of serializer cache is not bounded: this is something that goes against the cardinal rule of caches ("never use unbounded caches for unbounded domain").
This needs to go in 3.0 because tweaks to caching are generally high-risk changes.

[cowtowncoder]

One note: default maximum size is 4000, which should be sufficient for most uses.
Value could be made configurable (either explicitly, or by allowing cache instance to be constructed as part of building of mapper) but for now it is most important to add some bound.

[cowtowncoder]

Could consider backporting in 2.10, but would need more work with interaction between SerializerCache and ReadOnlyClassToSerializerMap.