Full command forwarding

[jumaffre]

This PR introduces support for full RPC forwarding if a Write command is issued on a follower.

Changes

• In rpcendpoint.h, the rpc response is no longer synchronously written to the session if the command requires forwarding. Instead, nothing is written to the session (i.e. the session stays open) until the forwarded response comes back from the leader.
• There is a new Forwarder object that is only known by enclave.h and the frontends that are allowed to forward (only the USERS one for now) and that deals with the appending/reading the appropriate headers to the message before being sent/received (encrypted).
• The previous CMake RPC_FORWARD_TO_LEADER is gone as forwarding is now enabled by default.

Notes

• Nothing is store in the enclave while a command is being forwarded. Instead, the session id of the client that initiated the forwarded command is passed to the leader and back to the follower so that the follower knows which client connection to send the forwarded response back to.
• This does not support daisy chaining, i.e. if a follower forwards a command to a leader and if, by the time the forwarded command is received by that leader an election has occurred and the leader has changed, the command is not re-forwarded to the new leader. Instead, the previous redirection error message with the IP/Port of the new leader is sent back. In practice, this is unlikely to happen.
• This new behaviour is currently tested by the e2e_logging and e2e_scenarios tests.

Still to do

• Write better unit tests for it [in progress now]
• Make the join protocol work with this new async behaviour
• Support redirection for all frontends (i.e. management, members and nodes)