Fix txpool dump/restore race condition

[fab-10]

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PR description

The disk access was not properly guarded against overlapping dump/restore tasks

Fixed Issue(s)

fixes #6664

[macfarla]

do we not need the equivalent of releasing in finally

[fab-10]

no and this was the error, since the operation is run async, we can't know here when it is done, and so it is not up to us to release the lock, but the lock must be released after the async operation is done with this code CompletableFuture.runAsync(operation).thenRun(diskAccessLock::release));