ssh: set rekeying thresholds on construction 
ssh: set rekeying thresholds on construction 

The normal handshake kicks off with a waitSession(), which guarantees
that we never attempt to send data before the first kex is completed,
but ensuring readPacketsLeft > 0 and writePacketsLeft > 0 helps
understand that thresholds can never cause spurious rekeying at the
start of a connection.

Change-Id: If5bcafcda0c7d16fd21f22c664101ac5f5b487d7
Reviewed-on: https://go-review.googlesource.com/38696
Reviewed-by: Adam Langley <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>
Run-TryBot: Adam Langley <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

ScriptRock/crypto,golang/crypto,ScriptRock/crypto,golang/crypto,golang/crypto