av-store: Move prune on a separate thread (#7263)

* av-store: Move prune on a separate thread

There are situations where pruning of the data could take more than a few
seconds and that might make the whole subsystem unreponsive. To avoid this just
move the prune process on a separate thread.

See: https://github.com/paritytech/polkadot/issues/7237

, for more details.

Signed-off-by: Alexandru Gheorghe <[email protected]>

* av-store: Add log that prunning started

Signed-off-by: Alexandru Gheorghe <[email protected]>

* av-store: modify log severity

Signed-off-by: Alexandru Gheorghe <[email protected]>

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Signed-off-by: Alexandru Gheorghe <[email protected]>