Use sun.misc.Unsafe to access a direct ByteBuffer

- Add PooledUnsafeDirectByteBuf, a variant of PooledDirectByteBuf, which
  accesses its underlying direct ByteBuffer using sun.misc.Unsafe.
- To decouple Netty from sun.misc.*, sun.misc.Unsafe is accessed via
  PlatformDependent.
- This change solely introduces about 8+% improvement in direct memory
  access according to the tests conducted as described in #918