acl-plugin: an elog-based tracing implementation for troubleshooting the conn cleaner...

acl-plugin: an elog-based tracing implementation for troubleshooting the conn cleaner threads interactions

This replaces some of the early-stage commented-out printf()s with
an elog-based debug collector.

It is aimed to be "better than nothing" initial implementation to be available
in the field. It will be refined/updated based on use. This initial code
is focused on the main/worker threads interactions, hence uses just
the worker tracks.

This code adds a developer debug CLI "set acl-plugin session table event-trace 1",
which allows to gather the events pertaining to connection cleaning.
The CLI is deliberately not part of the online help, as the express
declaration that the semantics/trace levels, etc. are subject to change
without notice.

Change-Id: I3536309f737b73e50639cd5780822dcde667fc2c
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>