index: v2: parallel by default

index: v2: parallel by default

InboxWritable should only set $v2w->{parallel} if the $parallel
flag is defined to 0 or 1.  We want indexing a new inbox to
utilize SMP, just like --reindex.

-index once again allows -j0/--jobs=0 to force single-process
use, and we'll be ensuring that works in tests to maintain
performance on small systems.

Fixes: 61a2fff5b34a3e32 ("admin: move index_inbox over")