Kfsio lib: add delegation token methods to decrypt session key with the specified key instead of searching keys in crypto keys. This is need to allow key decryption with on the client session key. Add key, key id and session key accessor methods to kfs net client state machine. Store session key, in the kfs net client state machine until new connection establishment.
Client lib: implement key decryption logic with ChunkAccessOp class. Keep track of chunk server and chunk access tokens expiration time, in order to update these with the write master before these expire. Always ask write master to update chunk access token when allocating write id, as the chunk access token returned by the write master should have much lager than lease timeout lifetime, as its subject is bound to the write id.