Allow persistent connections to be configured per database host

Summary: Ref T11044. Fixes T11672. In T11672, persistent connections seem to work fine, but they can require max_connections and other settings to be raised. Since most users don't need them, make them an advanced option.

Test Plan: Configured persistent connections, loaded some pages, observed persistent connections get used.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11044, T11672

Differential Revision: https://secure.phabricator.com/D16913