Force serialization always for `convertValue()`; avoid short-cuts

[cowtowncoder]

Various "optimizations" for ObjectMapper.convertValue() -- that is, short-circuit code that tries to figure out if it is possible to omit write+read handling altogether and simply return input value -- have caused much confusion, and even breakage for legit use cases (even if most cases have been resolved by more heuristics).

But it seems that best course would be to simplify things and simply always do write+read handling and let caller handle optimizations if ones are desired.

For backwards compatibility reasons let's do this for 3.0 however.

[cowtowncoder]

Actually. No, I think it will make sense to do this in 2.10.