mbserver: improve consistency and information density of logging
mbserver: improve consistency and information density of logging

In many cases, we must reconstruct "what has happened here?" scenarios from
mbserver log messages.  Unfortunately, if you're missing any data, e.g.
because syslog or systemd-journald decided you're too chatty and suppressed
your spammy messages, then it can be difficult to reconstruct state
properly.

So, change the logging to put out fewer lines, and pack more information
into each line we do emit.

- When changing a client's page count, put the new page count on the line
  (labeled "current").  This allows us to know how much the client has,
  even if we've lost some log lines.

- Quite a lot of the time, clients give up no memory.  Suppress the
  messages that only tell us that nothing changed.

- Add a "verbose" switch so we can get the chatty logging back when needed.

- Format messages more consistently.

srwalter/membroker,srwalter/membroker