R1:c059149eb98e

Remove Drydock host resource limits and give working copies simple limits

Summary:
Ref T9252. Right now, we have very strict limits on Drydock: one lease per host, and one working copy per working copy blueprint.

These are silly and getting in the way of using "Land Revision" more widely, since we need at least one working copy for each landable repository.

For now, just remove the host limit and put a simple limit on working copies. This might need to be fancier some day (e.g., limit working copies per…