JNI direct buffer function speedup, part 3.

JNI direct buffer function speedup, part 3.

This caches the effective address in a new field in the base Buffer
object.  The first time something calls through one of the various
getEffectiveAddress calls, the value is set.  (This seemed easier than
chasing down the constructors and factories, and also prevents bit rot
on the "slow path" in the VM.)