R1:f8c22252686e

Use persistent database connections from web contexts

Summary:
Ref T11672. Depends on D16577. When establishing a connection from a webserver context, try to use persistent connections.

The hope is that this will fix outbound port exhaustion issues experienced on repository hosts handling large queue volumes.

Test Plan:
Added this to a page:

```lang=php
$tables = array(
new PhabricatorUser(),
new ManiphestTask(),
new DifferentialRevision(),
new PhabricatorRepository()…