Distinguish log drop counts between internal and per-client.

If you're seeing drops, it would be nice to know what is slow.
Remove StreamLogger.size.

This value optimises for the wrong case: the case where the feature is
unused. If a feature is inefficient, the correct approach is not to
optimise for avoiding its use, but to resolve the inefficiency.

Additionally, the common case is for this feature to be used. We have
found it useful to have an always-on querylog. We have found it to be
reasonably efficient.

This "optimisation" was added with the first iteration of this feature
and likely falls into the category of premature optimisation.

TODO(plobsing): eliminate flakiness and sleepy races in the
streamlog_test.
Fixup streamlog test.

If we're more synchronous, we can be less flaky. ServiceManagement is
too heavyweight for what we're trying to accomplish in this test.

Also discovered that the ServeLogs handler doesn't return to the client
until it serves the first method (this broke a test rewrite). This has
been annoying me anyways on some low-traffic servers, so fixing that was
a nice pickup.
Add positive-result counters to streamlog.

It would be useful to consider drop counts as a fraction.
Merge pull request #96 from plobsing/per-subscriber-counts

Improvements to StreamLogger