The metadata agent currently runs with a default socket backlog
of 128. This isn't enough on a busy network node, even when
spawning multiple worker processes.
This change addes a new "metadata_backlog = XX" to the ini file
to support a configurable value to help improve performance.
Change-Id: Ibea398f3b65a56deb1418f39810d87d8360ea9f3
Closes-bug: #1274536