Avoid temp byte array creation when doing cacheDataOnWrite

code in HFileBlock.Writer.cloneUncompressedBufferWithHeader

ByteBuffer cloneUncompressedBufferWithHeader() {
  expectState(State.BLOCK_READY);
  byte[] uncompressedBlockBytesWithHeader = baosInMemory.toByteArray();
  …
}

When cacheOnWrite feature enabled, a temp byte array was created in order to copy block’s data, we can avoid this by use of ByteBuffAllocator. This can improve GC performance in write heavy scenarios.