lvutil: Remove SR lock during thin attach/detach

[benjamreis]

This lock is normally useless and can create a dead lock when thin mode is activated:

• A task tries to deactivate a volume during a VM shutdown on a slave (so a VDI A is locked). Then a new task is created on the master host, we try to get the SR lock on the master.
• In parallel a tap-pause is asked from the master to the slave, the master SR lock is now locked. The tap-pause request is received on the slave, but we can't lock VDI A because it's already locked.

So to summarize: a dead lock is only possible if we try to shutdown a VM with a particular VDI and if we try to snapshot it in the same time.

[benjamreis]

It is an issue we found out on our Linstor driver that also applies to this one.

Unless the lock is necessary to activate the LVM cache, in this case we can re add it only for the cache.

[MarkSymsCtx]

So to summarize: a dead lock is only possible if we try to shutdown a VM with a particular VDI and if we try to snapshot it in the same time.

This sounds like the solution would be for the initiating process on the coordinator to hold a lock on the VDI whilst performing these operations and not permit the detach and snapshot operations to occur in parallel, that sounds like an unending recipe for edge cases.