*index: more consistent graceful shutdown checks

*index: more consistent graceful shutdown checks

v1 and v2 inbox indexing now supports graceful shutdown checks
just like ExtSearchIdx.  Additionally, we'll consistently
perform quit checks at the top of loops for consistency.

Interaction with the --xapian-only and --sequential-shard
options are a bit lacking, and will warn the user to use
"--reindex --xapian-only" to fix.