R1:43cf4edfb16b

When waiting for long-running Harbormaster futures to resolve, close idle database connections

Summary:
Ref T13216. See PHI916. Harbormaster builds may be long-running, particularly if they effectively wrap `ssh ... ./run-huge-build.sh`. If we spend more than a few seconds waiting for futures to resolve, close idle database connections.

The general goal here is to reduce the held connection load for installs with a very large number of test runners.

Test Plan: Added debugging code to `phlog()` closures…