extsearchidx: close DB handles after use if FD constrained

Most distros ship with low RLIMIT_NOFILE limits and surprises
may lurk for admins who configure many inboxes.  Keep FD usage
under control to avoid EMFILE errors at inopportune times during
reindex.

From what I can tell, this is the only place where extindex can
have unpredictable FD growth when there's thousands of inboxes,
and it's in an extremely rare code path.

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