Peer-id patch v7

Added new packet format P_DATA_V2, which includes peer-id. If server
supports, client sends all data packets in the new format. When data
packet arrives, server identifies peer by peer-id. If peer's ip/port has
changed, server assumes that client has floated, verifies HMAC and
updates ip/port in internal structs.

Changes in v7:
A few nitpicks.

Changes in v6:
Fixed: Make sure float won't happen if hmac check failed (regression).
Fixed: Access outside of bounds of array, which has caused memory corruption and crash.
Various review fixes.

Changes in v5:
Protection agains replay attack by commiting float changes only after
existing packet processing flow has completed.

If peer floats to an address which is already taken by another active
session, drop float packet, otherwise disconnect existing session.

Changes in v4:
Handles correctly float to an address which is used by another peer.
This also has fixed crash on assert in multi_client_disconnect.

Changes in v3:
Bugfix: If float happens after TLS renegotiation and there are no
data packets between reneg and float, server will not recognize floated client.