[Searchable Snapshots] Fail searches if local disk cache is full

[andrross]

Related to #7033

At search time, if there are enough concurrent searches that there is no capacity remaining in the reserved disk cache, then searches should be rejected with an appropriate error message. The current behavior is that the system will exceed the provisioned disk cache size. Note that this only happens if the portion of the disk reserved for the local cache is too small for the workload, but the right behavior is probably to fail the searches as opposed to exceeding provisioned disk usage which can result in impact to the local indexes by taking more disk space for the remote searches than was provisioned.