x/build: coordinator should keep a small pool of ready-to-go GCE buildlets

[bradfitz]

The coordinator's GCE buildlet pool should always have a few of each type of buildlet ready to go to avoid the start-up latency.

e.g. an idle Plan 9, OpenBSD 32/63, FreeBSD 64 (at least 2x), Linux (probably at least 4x), etc.

Then once we start building them, we also start spinning up the helpers for test sharding.

This will raises costs, but probably shouldn't be too bad. I'll post some math on the cost here later.

/cc @adg

[bradfitz]

Related bug that'd probably need to happen about the same time: #19178