Add Multi functionality

This includes background workers, so that the consumer of the API need
not concern itself with any blocking calls like "PostMessages" in the
original Client implementation.

In the PostMessages had to be taught to return statistics to make this
come out nicely.

Signed-off-by: Daniel Farina <daniel@heroku.com>
Fix Client statistics gathering

The composition here was not thought out very hard.  Putting it in the
bundle type makes things cleaner and more obviously correct.  Also the
statistics functionality is also very obvious not tested, because the
first use of Multi failed miserably without these fixes.

Signed-off-by: Daniel Farina <daniel@heroku.com>
Rename Multi to "Client" and Client to "MiniClient"

The thinking here is that the Minimal Client might be useful to some
people, but most people are going to want Multi anyway, which is
capable of batching, dropping, and posting of its own volition.

So, with that thought in mind, rename Multi to be just "Client" so
that people don't write logplexc consumers that do normally
undesirable things, such as blocking in event of backlog.

Signed-off-by: Daniel Farina <daniel@heroku.com>