Change prefix search from O(N*M) to O(NlogM). The previous code

Change prefix search from O(N*M) to O(NlogM).  The previous code
linearly merged the doclists, so as the accumulated list got large,
things got slow (the M term, a fucntion of the number of documents in
the index).  This change does pairwise merges until a single doclist
remains.  A test search of 't*' against a database of RFC text
improves from 1m16s to 4.75s. (CVS 4599)

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