pgsql: Make sure that open hash table scans are cleaned up when bgwriter

Log Message:
-----------
Make sure that open hash table scans are cleaned up when bgwriter tries to
recover from elog(ERROR). Problem was created by introduction of hash seq
search tracking awhile back, and affects all branches that have bgwriter;
in HEAD the disease has snuck into autovacuum and walwriter too. (Not sure
that the latter two use hash_seq_search at the moment, but surely they might
someday.) Per report from Sergey Koposov.

Modified Files:
--------------
pgsql/src/backend/postmaster:
autovacuum.c (r1.56 -> r1.57)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c?r1=1.56&r2=1.57)
bgwriter.c (r1.41 -> r1.42)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/bgwriter.c?r1=1.41&r2=1.42)
walwriter.c (r1.1 -> r1.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/walwriter.c?r1=1.1&r2=1.2)