pgsql: Replace plain-memory ordered array by binary tree in ts_stat()

Log Message:
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Replace plain-memory ordered array by binary tree in ts_stat() function.
Performance is increased from 50% up to 10^3 times depending on data.

Modified Files:
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    pgsql/src/backend/utils/adt:
        tsvector_op.c (r1.17 -> r1.18)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/tsvector_op.c?r1=1.17&r2=1.18)