ingest: implement bulk mode

[peterbourgon]

The ingester can take an entire segment in a single write or series of writes, somehow. It can block until that segment has been successfully replicated to store nodes, and only then accept the next segment from the forwarder. This would provide the "most durable" mode of ingestion.

There are some tricky things here. One, plumbing through the necessary infrastructure to communicate to the forwarder that the segment has been successfully replicated. Two, deciding when and how to assign ULIDs. Three, thinking through the failure modes carefully.