Re-used prepared statement from fts3 cursor. Previously, each call to

Re-used prepared statement from fts3 cursor.  Previously, each call to
fulltextFilter() finalized any existing prepared statement and
prepared a new one.  In the case where idxNum has not changed, simply
reseting the statement suffices.  This provides an order of magnitude
speedup in incoming joins against docid. (CVS 5489)

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