pgsql: During Hot Standby, fix drop database when sessions idle.

Log Message:
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During Hot Standby, fix drop database when sessions idle.
Previously we only cancelled sessions that were in-transaction.

Simple fix is to just cancel all sessions without waiting. Doing
it this way avoids complicating common code paths, which would
not be worth the trouble to cover this rare case.

Problem report and fix by Andres Freund, edited somewhat by me

Modified Files:
--------------
pgsql/src/backend/commands:
dbcommands.c (r1.230 -> r1.231)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/dbcommands.c?r1=1.230&r2=1.231)
pgsql/src/backend/storage/ipc:
procarray.c (r1.54 -> r1.55)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/procarray.c?r1=1.54&r2=1.55)
pgsql/src/include/storage:
procarray.h (r1.28 -> r1.29)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/procarray.h?r1=1.28&r2=1.29)