delay perf_check runs until the server is actually ready for service

[aks]

putting '@PerfCheck' into a github PR causes a perf check to be run.

However, when the database that perf check uses is being backed up, perf-check reports spurious, random errors.

Perf check should either stall while it's database is being restored, and should not process github callbacks until it's actually ready for work or error in a more sane way.