all: tons of builder work

* reverse buildlet rework (multiplexed TCP connections, instead
  of a hacky reverse roundtripper)

* scaleway ARM image improvements

* parallel gzip implementation, which makes things ~8x faster on
  Scaleway.

* merge watcher into the coordinator, for easier deployments

Change-Id: I55d769f982e6583b261435309faa1f718a15fde1
Reviewed-on: https://go-review.googlesource.com/12665
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
28 files changed