* Remove redundant schedulable check in `FreeWriteClaims`. If a volume
has been created but not yet claimed, its capabilities will be checked
in `WriteSchedulable` at both scheduling time and claim time. We don't
need to also check them in the `FreeWriteClaims` method.
* Enforce maximum volume claims for writers.
When the scheduler checks feasibility for CSI volumes, the check is
fairly loose: earlier versions of the same job are not counted as
active claims. This allows the scheduler to place new allocations
for the new version of a job, under the assumption that we'll replace
the existing allocations and their volume claims.
But when the alloc runner claims the volume, we need to enforce the
active claims even if they're for allocations of an earlier version of
the job. Otherwise we'll try to mount a volume that's currently being
unmounted, and this will cause replacement allocations to frequently
fail.
* Enforce single-node reader check for read-only volumes. When the
alloc runner makes a claim for a read-only volume, we only check that
the volume is potentially schedulable and not that it actually has
free read claims.
