Timeline history files restored from archive not kept in pg_xlog, while WAL files are

The cascading replication patch made a change to the way WAL files 
restored from archive are handled. Since then, when a WAL file is 
restored from archive, it's copied under the correct filename to 
pg_xlog. Aside from supporting cascading replication, this has the 
advantage that if the archive subsequently goes offline, and the standby 
is restarted, it can still recover back up to the point where it was 
before. It also means that you can take an offline backup of the 
standby, and pg_xlog includes all the files required to restore.

However, timeline history files are still not retained. When a standby 
restores a timeline history file from the archive, it's restored under a 
temporary filename, and discarded after it's read. That means that if 
the latest checkpoint is on a WAL segment that includes an earlier 
timeline switch, you again need the archive to be online to restore from 
that, or you get an error like:

LOG:  unexpected timeline ID 1 in log file 0, segment 3, offset 0

This is a pre-existing issue in 9.2. In git master, it also means that 
if a standby follows a master through the archive, a cascading standby 
won't find the timeline history files in the 1st standby, and won't be 
able to follow timeline switches.

The straightforward fix is treat timeline history files the same WAL 
files, and copy them into pg_xlog when they're restored from the 
archive. Patch attached, and a test script I used to test this. Barring 
objections, I'll commit the patch tomorrow.

- Heikki