close FileChannel after mapping

Closing the channel shouldn't effect the buffer since it should be fully read into memory.

"Another issue is that if the Netty code is using the TachyonByteBuffer?"

The only thing `TachyonByteBuffer` does it call `mTachyonFS.unlockBlock(mBlockId, mBlockLockId);` when a user calls `close`, so not sure what it would offer here.