pgsql: Revise the TIDBitmap API to support multiple concurrent

Log Message:
-----------
Revise the TIDBitmap API to support multiple concurrent iterations over a
bitmap. This is extracted from Greg Stark's posix_fadvise patch; it seems
worth committing separately, since it's potentially useful independently of
posix_fadvise.

Modified Files:
--------------
pgsql/src/backend/access/gin:
ginget.c (r1.21 -> r1.22)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gin/ginget.c?r1=1.21&r2=1.22)
ginscan.c (r1.20 -> r1.21)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gin/ginscan.c?r1=1.20&r2=1.21)
pgsql/src/backend/executor:
nodeBitmapHeapscan.c (r1.31 -> r1.32)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeBitmapHeapscan.c?r1=1.31&r2=1.32)
pgsql/src/backend/nodes:
tidbitmap.c (r1.16 -> r1.17)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/tidbitmap.c?r1=1.16&r2=1.17)
pgsql/src/include/access:
gin.h (r1.27 -> r1.28)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/gin.h?r1=1.27&r2=1.28)
pgsql/src/include/nodes:
execnodes.h (r1.199 -> r1.200)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/execnodes.h?r1=1.199&r2=1.200)
tidbitmap.h (r1.8 -> r1.9)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/tidbitmap.h?r1=1.8&r2=1.9)