x/build: coordinator should have status page of last-seen buildlet versions

[bradfitz]

For debugging whether all buildlets are running the latest versions, it would be nice if we could go to some new debug URL, like:

http://farmer.golang.org/buildlets

And see for each GOOS-GOARCH, the last-seen buildlet version number.

For each build, the buildlet could include its GOOS-GOARCH, build-date, go rev, x/build rev, in some HTTP response header, or in its existing /status JSON reply, and then the buildlet could add it to a global map guarded by some new (or existing statusMu) mutex.

Then we'd be able to quickly debug that a buildlet is out of date.

/cc @crawshaw @adg

[bradfitz]

This shows up on the reverse buildlet status nowadays, which I use periodically. No need for specialized page.