These changes should be a fix for both issues:
#1451
#1457

This adds support for crutest to use a provided dsc endpoint to construct
a Volume object.

Moved the existing volume creation steps to a new function, and added another
option on how we can create a Volume.  The two previous ways of creating
a volume are not changed (though I changed a log message and added some
warnings).  The new code is in taking the dsc provided endpoint and
using that to construct a volume.

Additional dsc changes were made to help provide Volume info.

Renamed things in dsc to better reflect what information they hold.
Specifically, update a bunch of region set comments, as dsc just controls
crucible-downstairs processes, and does not know which ones are part of
what region set.

New dsc commands:
get_ds_uuid: Returns the UUID for the given client ID.
all_running: Returns true if all downstairs that dsc knows about are currently
in Running state.
get_region_count: Returns the total number of regions that dsc knows about.

New dsc behavior.
dsc will now wait on all downstairs starting before taking any commands.
The ability for dsc to answer a request can be used by a test to confirm that
all downstairs had started.
Add the ability to supply a dsc endpoint to crutest-cli
#1459

tools/test_replay.sh transitioned to using the new --dsc option, as that
test already required a dsc endpoint and was using a hard coded default
value for it.

tools/test_restart_repair.sh was updated to wait for dsc to report that
all downstairs are online after a restart.  This avoids a race where
we told dsc to start, and then start crutest, but the downstairs are not
yet online.

All the tests that use dsc will eventually transition to using it to
construct a Volume, but I'm pushing that work to another PR.

There are more changes coming, specifically:
Updating replace-before-acive and replace-reconcile to use the new
dsc option correctly instead of a default.
Updating tools/test_* to not require targets for crutest and instead use
dsc option.
More updates in crutest to update the current RegionInfo struct to be
aware of multiple sub-volumes.
#1454

Possibly some updates to the BlockIO trait.
#1455

Other work this enables
Making tests that use multiple sub-volumes.
Layering the current set of tests in a way so we can run the same tests

with a single sub-volume and with multiple sub-volumes