crimson/.../store-bench: refactor arguments and workloads

crimson/.../store-bench: refactor arguments and workloads

- Adds a workload abstraction grouping arguments with associated
  workload.
- Reworks argument parsing to occur prior to seastar app, allows
  passing unparsed arguments to ceph.
- Refactors time usages as necessary to use std::chrono types.
- Removes LOG_PREFIX usages that don't currently have log lines.
- Other minor cleanups not worth separating out.

Signed-off-by: Samuel Just <sjust@redhat.com>