Change metadata-agent to spawn multiple workers

There is currently only one metadata-agent per network node,
which could be handling connections from hundreds or thousands
of metadata-namespace-proxy processes.

This change addes a new "metadata_workers = XX" to the ini file
to support creating more workers to help improve performance.

Change-Id: Ib9ebcfc543a83982dd93db79c7dc631283fd3bfa
Partial-bug: #1274536
Change metadata-agent to have a configurable backlog

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
Merge "Change metadata-agent to spawn multiple workers"
Merge "Change metadata-agent to have a configurable backlog"