pgsql: Fix performance issues in replace_text(), replace_text_regexp(),

Log Message:
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Fix performance issues in replace_text(), replace_text_regexp(), and
text_to_array(): they all had O(N^2) behavior on long input strings in
multibyte encodings, because of repeated rescanning of the input text to
identify substrings whose positions/lengths were computed in characters
instead of bytes. Fix by tracking the current source position as a char
pointer as well as a character-count. Also avoid some unnecessary palloc
operations. text_to_array() also leaked memory intracall due to failure
to pfree temporary strings. Per gripe from Tatsuo Ishii.

Modified Files:
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pgsql/src/backend/utils/adt:
varlena.c (r1.152 -> r1.153)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/varlena.c.diff?r1=1.152&r2=1.153)