Set output and input encodings separately at end of KEX

[tomaswolf]

Once SSH_MSG_NEWKEYS is sent any subsequent packet sent must use the
new encoding settings. Once SSH_MSG_NEWKEYS is received, the new
encoding settings are to be used for any message received. So set the
cipher/mac/compression separately for outgoing and incoming messages
in sendNewKeys() and handleNewKeys().

Previously, we set both only in handleNewKeys(), i.e., when the peer's
SSH_MSG_NEWKEYS was received. This makes implementing a KEX extension
handler more complicated than necessary since it had to delay sending
the SSH_MSG_EXT_INFO packet until after the peer's SSH_MSG_NEW_KEYS was
received.

RFC 8308 recommends that "the server sends its SSH_MSG_EXT_INFO not
only as the next packet after SSH_MSG_NEWKEYS, but without delay". This
is now possible since the output settings are already set up correctly.

SSH_MSG_EXT_INFO is always sent and received after SSH_MSG_NEWKEY, and
the Apache MINA sshd implementation guarantees that either party handles
the peer's SSH_MSG_NEWKEY after having sent its own SSH_MSG_NEWKEY.

[tomaswolf]

@benhumphreys since this changes sending of server-sig-algs and you apparently want to use that in Bitbucket Server you might want to double-check this change, too. (It fixes the remaining issue of sending the EXT_INFO message "without delay" as recommended by RFC 8308.)

[benhumphreys]

Thanks @tomaswolf. I've also tested this branch and have found no problems.
I expanded my testing a little to include OpenSSH 6.6.1p1, 8.1p, 8.2p1 and 8.8p1.

Thanks again!