Implement client-initiated rekeys after an hour, or after 1Gb of

Implement client-initiated rekeys after an hour, or after 1Gb of
data transfer in either direction (whichever comes first), or at
explicit client request (nice idea Jacob). Have tested by lowering
the limits, and it all seems solid enough; in particular, this has
also allowed me to test the behaviour when connection-level data is
received during rekey, and that looks fine too (at least it does
_now_ :-).

git-svn-id: http://svn.tartarus.org/sgt/putty@4908 cda61777-01e9-0310-a592-d414129be87e