verifier: serialize the fetching of blocks within a share so that we don't use too...

verifier: serialize the fetching of blocks within a share so that we don't use too much RAM

Shares are still verified in parallel, but within a share, don't request a
block until the previous block has been verified and the memory we used to hold
it has been freed up.

Patch originally due to Brian. This version has a mockery-patchery-style test
which is "low tech" (it implements the patching inline in the test code instead
of using an extension of the mock.patch() function from the mock library) and
which unpatches in case of exception.

fixes #1395