Pause buffer fill on push (#301)

* Prevent current buffer from overfilling while writes complete

* Send bulk updates in parallel

* Move thread pool out of context

Simplify DBUpdatesBuffer threading (#300)

* MongoIngester: expose buffer parameters

* Actually parallelize bulk write

* Delegate push to thread

* Promote push interval to float

AmpelLogger: support stack_info, stacklevel kwargs (#299)

Useful for logging in context managers

Encapsulate setup/teardown in context managers (#297)

There are lots of places where some setup needs to happen before a
code block, and some action needs to happen afterwards:

- DBUpdatesBuffer: start/stop pusher thread, flush buffered updates
- AmpelLogger: flush buffered records
- AbsWorker and co: set current run id, clear cached unit instances
- AbsConsumer: subscribe/unsubscribe from queue
- AbsProducer: flush buffered messages

While all of these can be handled by try/finally blocks, this puts
onus on the user to clean up afterwards.  Use context managers to
encapsulate these steps in the objects themselves.

Pass extra args to IngestionWorker (#296)

Add IngestionWorker, factor MongoStockUpdater into AbsIngester (#295)

* Add IngestionWorker, a worker that ingests

* Factor MongoStockUpdater interface into AbsIngester

Add a null producer for testing (#294)

Skip already run T2 docs (#293)

Move grouping context out of ChainedIngestionHandler (#292)

* Move grouping context out of ChainedIngestionHandler

* Bump version to 0.10.6a2

* Move ack inside context

Batch acknowledgments in AbsConsumer (#291)

* Skip datapoint retention if not implemented

* Acknowledge in chunks

* Verify ack batching

* Bump version to 0.10.6a1