Move connection limiting from gerrit's Jetty to Apache

Gerrit's Jetty floods logs with "Dispatched Failed" messages, once its
buffer for queued client connections gets full. Hence, we shift
connection limiting to apache and thereby keep Jetty's queued
connection buffers empty.

As we currently do not set gerrit's httpd.maxThreads, and that setting
comes with a 25 default, we limit apache to pass at most that many
connections to gerrit.

For now, we should never hit the limit of 500 queued connections for
gerrit, as apache only forwards 25 connections. We nevertheless don't
set httpd.maxQueued to 0, just to provide some guarding in case we
accidentally set up a gerrit instance that's directly hooked to the
net.

Bug: 47284
Change-Id: I8fd7aff8e34c11b975dadf85191da51fd700dc94