Improve DBEs read performance by avoiding byte array deep copies for key[] and value[]

As part of HBASE-10801, we tried to reduce the copy of the value [] in forming the KV from the DBEs.
The keys required copying and this was restricting us in using Cells and always wanted to copy to be done.
The idea here is to replace the key byte[] as ByteBuffer and create a consecutive stream of the keys (currently the same byte[] is used and hence the copy). Use offset and length to track this key bytebuffer.
The copy of the encoded format to normal Key format is definitely needed and can't be avoided but we could always avoid the deep copy of the bytes to form a KV and thus use cells effectively. Working on a patch, will post it soon.