Add script to purge changed pages from cache

This maintenance script could be used to recover from large HTCP
listener outages and/or network partitions. It searches the database for
pages having revisions occurring within a specified time period,
converts them to full URLs and sends squid/varnish purge messages using
SquidUpdate. A command line option allows specifying a specific
IP:port combination for HTCP purge datagrams which can be used to
isolate the purge announcement to a particular varnish server or data
center.

Co-Authored by: Aaron Schulz <aschulz@wikimedia.org>
Bug: 54647
Change-Id: Id0e95290e83333545e6bb0378cc620c35c653bf9
(cherry picked from commit 7cee9fd38dbc7c2f33540ec55755c8d8c2155fac)