Logging WAL when updating hintbit

Hi all,

I attached patch adds new wal_level 'all'.
If wal_level is set 'all', the server logs WAL not only when wal_level
is set 'hot_standby' ,but also when updating hint bit.
That is, we will be able to know all of the changed block number by
reading the WALs.
This wal_level is infrastructure for fast failback. (i.g., without fresh backup)
It need to cooperate with pg_rewind.

Not only that, I think it will be profitable infrastructure for
differential backup.
And it leads to improve performance at standby server side. Because
the standby server doesn't update hintbit by itself, but FPW is
replicated to standby server and applied.

It is very simple patch, server writes FPW at same timing as when
checksum is enabled. i.g., just without calculate checksum.

Discussion of Fast failback is here
<http://www.postgresql.org/message-id/CAF8Q-Gy7xa60HwXc0MKajjkWFEbFDWTG=gGyu1KmT+s2xcQ-bw@mail.gmail.com>

Regards,

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Sawada Masahiko