Winery: Implement asynchronous packing

Highlights of this pile of changes, in commit order:

• avoid double-init() in WineryWriter (would create new shards and never use them)
• replace python-sh with direct calls to subprocess
• some hardening of the test fixtures for postgresql and ceph rbd so that they teardown properly in "all" (famous last words) cases
• cleaner separation of parameters in the benchmark code (between ro workers and rw workers)
• implement shard lifecycle with an enum instead of a pair of booleans
• add an asynchronous packing function
• hook the asynchronous packing into benchmarks

A few commits to improve mypy coverage have been sprinkled throughout.

TODO:

• add a CLI entry point for packing
• future: get rid of all the Winery classes passing around bare kwargs dicts.

Implements #4741 (closed)