extindex: fix --reindex

extindex: fix --reindex

`public-inbox-extindex --reindex' deprioritizes itself for
public-inbox-extindex invocations without --reindex by shutting
down shard processes to let other processes acquire the lock and
process new messages, first.

Restarting shard processes during --reindex was causing new
Xapian shards to be written to v2 inboxes instead of the
extindex itself.  This bug was introduced with the
simplifications to internal data structures to eliminate the
ad-hoc $sync structure.

The local-ized use of ExtSearchIdx->{ibx} tricked
PublicInbox::SearchIdxShard::new into using the standard v2 code
path.  So make SearchIdxShard->new check the `$v2w' object for
the ability to call `eidx_sync' rather than the existence of the
{ibx} field.

I only noticed this bug while working on the --split-shards
feature for performance.

Fixes: 922b765d ((ext)index: move {max_size} and related bits to $self, 2025-01-10)