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, saw connections closed while running builds.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13216

Differential Revision: https://secure.phabricator.com/D19824