vinyl: dump primary index after secondary

If a space has more than one index, appending a statement to it requires
reading the primary index to get the old tuple and delete it from
secondary indexes. This means that on local recovery from WAL, the
primary index must not be ahead of secondary indexes of the same space,
i.e. it must be dumped last. To assure that, do the following:

 - Make the dump heap prioritize indexes with greater iid if they are
   equally old.

 - Temporarily remove the primary index from the scheduler while a
   secondary index is being dumped.

Since every complete dump is consistent in vinyl, removal of a primary
index from the dump heap won't make the scheduler pick an already dumped
secondary index again, resulting in a live lock. It also guarantees that
while a primary index is exempted from the scheduler, it can't "run
ahead" of dumped secondary indexes, because only statements that had
been inserted by the start of the dump will be dumped, no matter what.

Closes #2410