Add snapshot stress tests (#78611)

Adds `SnapshotStressTestsIT`, a test that performs a wide variety of
concurrent snapshot-related operations to explore the corners of the
snapshot state machine in a randomized fashion:

- indexing docs, deleting and re-creating the indices
- restarting nodes
- removing and adding repositores
- taking snapshots (sometimes partial), cloning them, and deleting them

It ensures that these operations should succeed via a system of
shared/exclusive locks. None of the operations block. If the necessary
locks aren't all available then the operation just releases the ones it
has acquired and tries again later. The test completes after completing
a certain number of snapshots or after a certain time has elapsed.

Backport of #78596

Author: DaveCTurner