Fix OOB read in cdbcomponent_getComponentInfo() with partially registered mirrors

cdbcomponent_getComponentInfo() assumed that segment_db_info holds either
exactly one entry per content (no mirrors) or exactly two (fully mirrored)
and indexed the array with 2 * contentId in the latter case.  That
assumption breaks while mirrors are being registered one by one:
gpinitsystem's REGISTER_MIRRORS commits one gp_add_segment_mirror() per
content, so for several seconds gp_segment_configuration holds N
primaries plus k < N mirrors.

Any dispatching process that reads the catalog in that window computes an
index past the end of the palloc'd array.  The dtx recovery bgworker does
exactly that: every loop iteration it dispatches to all segments and then
destroys its component cache via DisconnectAndDestroyAllGangs(), so the
next iteration re-reads the half-populated catalog and crashes with
SIGSEGV.  The crash makes the postmaster set CleanupBackends, the restarted
dtx recovery immediately runs TerminateMppBackends(), crashes again, and
the coordinator loops forever producing core files, while gpinitsystem has
already failed with "failed to register mirror".  Larger clusters widen
the race window, which is why this surfaced on a 94+ segment cluster.

Replace the index arithmetic with a lower-bound binary search on segindex
over the sorted array, then pick the entry whose role matches what the QD
wants (active primary for a regular QD, mirror for a hot standby QD),
falling back to the last entry of the content as before.  Contents that do
not have a mirror yet are simply treated as single-copy.  All other
consumers of segment_db_info already iterate and filter by role, and FTS
already skips primaries without a peer, so this was the only place that
relied on the 2N layout.  Both the internal-FTS and external-FTS copies of
the function now share the new helper.

Verified on a demo cluster by deleting one mirror row in utility mode:
unpatched 2.1.0 reproduces the dtx recovery crash loop, patched main maps
every content to its own primary and "DTM Started" normally.

Fixes #2101