Make Netty channel bytebuf allocator configurable

Netty supports different strategies for allocating byte buffers for IO and channel modules. We do not allow the site operator to fine tune this but could. It would be particularly useful to allow preference of heap buffers where direct memory may be limited or utilized for other purposes.

Support site configuration of the bytebuf allocator that Netty will use for NettyRpcServer channels. Property name is 'hbase.netty.rpcserver.allocator'. Default is no value, which is equivalent to "pooled". Valid values are:

• "pooled": use PooledByteBufAllocator
• "unpooled": use UnpooledByteBufAllocator
• "heap": use HeapByteBufAllocator, which is a PooledByteBufAllocator that preferentially allocates buffers on heap wherever possible
• <class>: If the value is none of the recognized labels, treat it as a class name implementing org.apache.hbase.thirdparty.io.netty.buffer.ByteBufAllocator. This allows the user to add a custom implementation, perhaps for debugging.