Add back accidentally removed JsonStringEncoder related methods in BufferRecyclers (like getJsonStringEncoder())

[cowtowncoder]

A few methods were added in 2.9.x for BufferRecyclers, for more efficient use of JsonStringEncoder. Unfortunately those methods did have unnecessary complexity and ended up being removed from 2.10.
The issue is that while these were intended for internal-only use, they were public, and some libraries/frameworks ended up using them.

Since it is easy enough to add them back in 2.12 patch (for 2.12.5), let's do that: mark methods as @Deprecated, remove from 3.0 -- the way it should have been done to begin with, wrt 2.10.
I will merge change itself in 2.10 branch, just in case we might end up with a patch, but the important version is 2.12.5 as that will definitely be released.