x/build: make GCE buildlet pool quota accounting smarter

[bradfitz]

The GCE implementation of BuildletPool's quota accounting is way too dumb & conservative.

If it counted better, it could push our quota limits a lot harder.

In fact, it could push things as hard as possible until it hit limits, learn the limits, and then count from there.

Or it could poll quotas? (is there an API?)

Currently we have quota for 200 CPUs (and N RAM and disk, etc) but we limit ourselves to 60 builds at once, just because we haven't done the work to count how much of resource $X each build uses.

/cc @adg @crawshaw