Awa_mirage: Fix rekey

[reynir]

The application can experience a denial of service if a user sends for example a bunch of backspaces to the shell. It seems this creates a bunch of pending timeouts that eventually slows down the server to a grind. The Lwt.catch call was at best useless.

[hannesm]

I approve this PR - just for common understanding, what happens in nexus:

• this is the main loop of the server for each client, which keeps three promises:
• it reads from the network (net_read),
• the Lwt_switch (to terminate the connection switched_off),
• and waits for a key tmeout (since keys may need to be rekeyed -- earlier timeout, now rekey_promise)

In the code, the Lwt.nchoose_split is used since multiple promises may be fulfilled at any given time. (loop handles each fulfilled promise). When a Timeout is received (Rekey event), the rekey is done, and a fresh promise is inserted.

The handling of network reads is a bit special: a separate buffer, input_buffer, is passed to nexus (initially empty) -- and this is being decoded / decrypted, and may accumulate more fragmented data (before a full ssh message is received).