Membership auto-verify error because of delay

[jllucas]

Membership auto-verify performs 2 actions:

• ask QED Log for a membership proof, which contains (among others) the version of the last event inserted.
• ask Snapshot Store for information about this last event.

Step 2 gives the error "version not found".

This is because, in high-load scenarios, publishers have certain delay in publishing snapshots to the Snapshot store. And "membership auto-verify" operation is faster than publishers.

So we need to implement some kind of retry system in order to auto-verify proofs in high-load scenarios.