cmd/coordinator, buildlet: kill TCP conn of misbehaving reversePool buildlets

Also: heartbeat reverse buildlets more often
Also: adjust cmd/go test expected duration.

Change-Id: I75fbb66a0b20ac7357ad7cf78fc545101ac9aa33
Reviewed-on: https://go-review.googlesource.com/10884
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>