timers: always delay handshakes for responder

With the prior behavior, when sending a packet, we checked to see if it
was about time to start a new handshake, and if we were past a certain
time, we started it. For the responder, we made that time a bit further
in the future than for the initiator, to prevent the thundering herd
problem of them both starting at the same time. However, this was
flawed.

If both parties stopped communicating after 2.2 minutes, and then one
party decided to initiate a TCP connection before the 3 minute mark, the
currently open session would be used. However, because it was after the
2.2 minute mark, both peers would try to initiate a handshake upon
sending their first packet. The errant flow was as follows:

1. Peer A sends SYN.
2. Peer A sees that his key is getting old and initiates new handshake.
3. Peer B receives SYN and sends ACK.
4. Peer B sees that his key is getting old and initiates new handshake.

Since these events happened after the 2.2 minute mark, there's no delay
between handshake initiations, and problems begin. The new behavior is
changed to:

1. Peer A sends SYN.
2. Peer A sees that his key is getting old and initiates new handshake.
3. Peer B receives SYN and sends ACK.
4. Peer B sees that his key is getting old and schedules a delayed
   handshake for 12.5 seconds in the future.
5. Peer B receives handshake initiation and cancels scheduled handshake.

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