Optimize PooledByteBufAllocator

Motivation:

We still have a room for improvement in PoolChunk.allocateRun() and
Subpage.allocate().

Modifications:

- Unroll the recursion in PoolChunk.allocateRun()
- Subpage.allocate() makes use of the 'nextAvail' value set by previous
  free().

Result:

- PoolChunk.allocateRun() optimization yields 10%+ improvements in
  allocation throughput for non-subpage allocations.
- Subpage.allocate() optimization makes the subpage allocations for
  tiny buffers as fast as non-tiny buffers even when the pageSize is
  huge (e.g. 1048576) because it doesn't need to perform a linear search
  in most cases.