Add fd event support to EventTaskRunner

This change allows applications to perform
general I/O completion handling as part of the
Run() loop, rather than just timeout completion.

Change-Id: Idce97a33f7a35348ac5a7d226491f9907edc3405
Reviewed-on: https://weave-review.googlesource.com/1447
Reviewed-by: Vitaly Buka <vitalybuka@google.com>