pgsql: First part of refactoring of code for ResolveRecoveryConflict.

Log Message:
-----------
First part of refactoring of code for ResolveRecoveryConflict. Purposes
of this are to centralise the conflict code to allow further change,
as well as to allow passing through the full reason for the conflict
through to the conflicting backends. Backend state alters how we
can handle different types of conflict so this is now required.
As originally suggested by Heikki, no longer optional.

Modified Files:
--------------
    pgsql/src/backend/access/heap:
        heapam.c (r1.281 -> r1.282)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c?r1=1.281&r2=1.282)
    pgsql/src/backend/access/nbtree:
        nbtxlog.c (r1.57 -> r1.58)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtxlog.c?r1=1.57&r2=1.58)
    pgsql/src/backend/commands:
        dbcommands.c (r1.231 -> r1.232)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/dbcommands.c?r1=1.231&r2=1.232)
        tablespace.c (r1.71 -> r1.72)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablespace.c?r1=1.71&r2=1.72)
    pgsql/src/backend/storage/ipc:
        standby.c (r1.3 -> r1.4)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/standby.c?r1=1.3&r2=1.4)
    pgsql/src/include/storage:
        standby.h (r1.2 -> r1.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/standby.h?r1=1.2&r2=1.3)