Change JsonFactory.builder() configuration of RecyclerPool to avoid allocation default implementation (in 3.0)

[cowtowncoder]

Currently JsonFactoryBuilder takes directly RecyclerPool<BufferRecycler> for configuration (as well JsonFactory) in 2.x; and the default implementation is eagerly assigned. For the case where this is changed, we allocate default implementation only to discard it.
This is necessary since 2.x does not actually have real builder (only facade over JsonFactory).
But for 3.0 we can change things to leave pool for builder to be null, which is only changed to default implementation when one is actually needed. This avoids unnecessary allocation.

Alternatively if this didn't work we could also consider Provider style configuration (which would have other benefits with serializability); but only do that if null doesn't work.