minor cleaning
updated TODO
first implementation of an asynchronous get interface for shardcache clients

NOTE: The signature is actually ignored when returning the data back to the
      clients in chunk. With the actual implementation of the protocol it's
      not possible to validate the signature until the end of the message is
      is reached. To achive a proper asynchronous validation the protocol should
      be extended to allow signing each single chunk of the communication
      instead of the message as a whole.

      A possible extension which would preserve the compatibility with older
      clients is to introduce a new signature header (0xF1) which would tell
      the receiver that chunked signing is being applied instead of the normal
      signature trailer for the whole message.

      So, while a normal signed message looks like :

      <SIG_HDR><HDR><SIZE><CHUNK>[<SIZE><CHUNK>...]<EOR><EOM><SIG>
        0xF0   |--------------------------------------------|
                                  SIGNED

      A message using the new version of the signature (so a response to a
      get_async command) would look like:

      <SIG2_HDR><HDR><SIZE><CHUNK><SIG>[<SIZE><CHUNK><SIG>...]<EOR><EOM><SIG>
         0xF1   |----------------|      |-----------|         |--------|
                      SIGNED                SIGNED              SIGNED