Implemented UPDATE for full-text tables.

Implemented UPDATE for full-text tables.

We handle an UPDATE to a row by performing an UPDATE on the content table and by building new position lists for each term which appears in either the old or new versions of the row.  We write these position lists all at once; this is presumably more efficient than a delete followed by an insert (which would first write empty position lists, then new position lists). (CVS 3434)

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