Defer flushes (fixes #276)

[jchambers]

This change defers calls to flush until either:

1. a given number of notifications are waiting to be flushed or
2. the channel has been idle for a given amount of time

This should significantly cut down on the number of calls to flush, which should yield significant performance benefits for users sending lots of notifications. These changes are easiest to read with whitespace-only changes hidden.

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TODO:

• Update docs (9f0ea27)