Key exchange is slow/gets timeouts

[cy384]

Key exchange is slow. This is arguably more of a bug in mbedtls but I want to track it here for my purposes. Notes:

• diffie-hellman-group1-sha1 key exchange method is too slow to run on (some? all?) 68k machines
• implementations in bignum.c in mbedtls, revisit mbedtls_mpi_exp_mod specifically
• figure how to detect when the remote host drops us due to timeout and tell the user
• figure out and document the minimum specs for the various methods
• is there some keepalive flag or message, either OT/TCP or in ssh/libssh2 that we can be using to prevent timeouts?

[Miles Raymond]

Is there debug output to see what cipher/key exchange the client is using to connect?