To mitigate bug in revdial that's retaining way too much memory and
causing coordinator evictions.

Also double memory limit for now.

Updates golang/go#31639

Change-Id: I19b6df92df9905b087c29884140096946037fa38
Reviewed-on: https://go-review.googlesource.com/c/build/+/173517
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

The old revdial has a simple multiplexing protocol that was like
HTTP/2 but without flow control, etc. But it was too simple (no flow
control) and too complex. Instead, just use one TCP connection per
reverse dialed connection. For now, the NAT'ed machine needs to go
re-connect for each incoming connection, but in practice that's just
once.

The old implementation is retained for now until all the buildlets are
updated.

Updates golang/go#31639

Change-Id: Id94c98d2949e695b677531b1221a827573543085
Reviewed-on: https://go-review.googlesource.com/c/build/+/174082
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>