Fix append queue telemetry and parameterize sizes.

The original append queue telemetry never worked, because it was
updated only upon the exit of the select statement, which would
usually liberate the queues of contents.  This has been fixed to
be reported arbitrarily.

The queue sizes are now parameterizable via flags.
Merge pull request #149 from prometheus/fix/telemetry/queue-reporting

Fix append queue telemetry and parameterize sizes.