Optimize array allocation by JsonStringEncoder

[cowtowncoder]

Looks like encoding methods in JsonStringEncoder were simplified in 2.10 to reduce likelihood of strange concurrency challenges (basically removing use of ThreadLocal). This was deemed not to be particularly performance/memory sensitive since methods are only used from one place, SerializedString.

One aspect that seems sub-optimal is the eager allocation of initial char[] / byte[] output buffer: 120 chars / 200 bytes.
Since the values most likely encoded are names, which tend to be a bit shorter, this is bit wasteful.
It may or may not be an actual concern for Jackson but seems like something easy enough to fix.

A simple change would be to shrink default lengths to something like 30 / 60 (for example), but given that we have information on actual input String length, we should be able to do better and calculate buffer that is likely sufficient.

[cowtowncoder]

Improved this properly for 2.13; 2.11/2.12 just get lower starting lengths (which should likely reduce overhead anyway), if new patch releases are made.