Here we add a worker class comparable to audit.Worker, which will be
responsible for pulling items off of the reverification queue and
calling reverifier.ReverifyPiece on them.
Note that piecewise reverification audits (which this will control) are
not yet being done. That is, nothing is being added to the
reverification queue at this point.
Refs: https://github.com/storj/storj/issues/5251
Change-Id: I94e28830e27caa49f2c8bd4a2336533e187ab69c
