pgsql: Make heap_fetch API more consistent by having the buffer remain

Log Message:
-----------
Make heap_fetch API more consistent by having the buffer remain pinned
in all cases when keep_buf = true. This allows ANALYZE's inner loop to
use heap_release_fetch, which saves multiple buffer lookups for the same
page and avoids overestimation of cost by the vacuum cost mechanism.

Modified Files:
--------------
pgsql/src/backend/access/heap:
heapam.c (r1.179 -> r1.180)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c.diff?r1=1.179&r2=1.180)
pgsql/src/backend/access/nbtree:
nbtinsert.c (r1.117 -> r1.118)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtinsert.c.diff?r1=1.117&r2=1.118)
pgsql/src/backend/commands:
analyze.c (r1.77 -> r1.78)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/analyze.c.diff?r1=1.77&r2=1.78)