misc/dashboard/builder: synchronize accesses to goroot, always -commit

misc/dashboard/builder: synchronize accesses to goroot, always -commit

This prevents the occasional issue when Mercurial screws up the locking
itself, and by moving the locking into this process we can use the
goroot for other things (such as automatically updating the builder
binary).

It also asks all builders to poll for new commits.

R=bradfitz, dave, minux.ma, rsc
CC=golang-dev
https://golang.org/cl/7178046