Implement new IBatchedLogEventSink interface

[bcallaghan-et]

I propose modifying the EmailSink so that it implements the IBatchedLogEventSink interface rather than deriving from the PeriodicBatching class. Both are from the same package, so no new dependencies are introduced. The introduction of this interface would allow the EmailSink to be used with other batching algorithms, rather than just time-sliced batching. I will volunteer to make this change, if others find it has value.

[bcallaghan-et]

My team found it useful, so I started making a prototype that we can use internally, which you can find here.

[nblumhardt]

Thanks again for raising this; closing as stale.