aggregators: refactor, introduce Writer

Introduce a Writer type, which encapsulates
a pebble.Batch and has methods for writing
metrics to the batch. Splitting this out of
the Aggregator type enables clients to create
independent Writers which are bound to an
execution context, to naturally avoid lock
contention.

Replace Aggregator.Stop with Aggregator.Close,
which should always be called, even if periodic
harvesting is never started.

Replace Aggregator.Run with Aggregator.StartHarvesting,
which starts the harvest loop in the background
with no context; it will be stopped by Aggregator.Close.

Move cachedEventsMap to its own file.