pgsql: Create a routine PageIndexMultiDelete() that replaces a loop

Log Message:
-----------
Create a routine PageIndexMultiDelete() that replaces a loop around
PageIndexTupleDelete() with a single pass of compactification ---
logic mostly lifted from PageRepairFragmentation. I noticed while
profiling that a VACUUM that's cleaning up a whole lot of deleted
tuples would spend as much as a third of its CPU time in
PageIndexTupleDelete; not too surprising considering the loop method
was roughly O(N^2) in the number of tuples involved.

Modified Files:
--------------
pgsql/src/backend/access/nbtree:
nbtpage.c (r1.81 -> r1.82)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtpage.c.diff?r1=1.81&r2=1.82)
nbtxlog.c (r1.19 -> r1.20)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtxlog.c.diff?r1=1.19&r2=1.20)
pgsql/src/backend/storage/page:
bufpage.c (r1.62 -> r1.63)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/page/bufpage.c.diff?r1=1.62&r2=1.63)
pgsql/src/include/storage:
bufpage.h (r1.63 -> r1.64)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/bufpage.h.diff?r1=1.63&r2=1.64)