Refactoring standby mode logic

The code that reads the WAL from the archive, from pg_xlog, and from a 
master server via walreceiver, is quite complicated. I'm talking about 
the WaitForWALToBecomeAvailable() function in xlog.c. I got frustrated 
with that while working on the "switching timeline over streaming 
replication" patch.

Attached is a patch to refactor that logic into a more straightforward 
state machine. It's always been a kind of a state machine, but it's been 
hard to see, as the code wasn't explicitly written that way. Any objections?

The only user-visible effect is that this slightly changes the order 
that recovery tries to read files from the archive, and pg_xlog, in the 
presence of multiple timelines. At the moment, if recovery fails to find 
a file on current timeline in the archive, it then tries to find it in 
pg_xlog. If it's not found there either, it checks if the file on next 
timeline exists in the archive, and then checks if exists in pg_xlog. 
For example, if we're currently recovering timeline 2, and target 
timeline is 4, and we're looking for WAL file A, the files are searched 
for in this order:

1. File 00000004000000000000000A in archive
2. File 00000004000000000000000A in pg_xlog
3. File 00000003000000000000000A in archive
4. File 00000003000000000000000A in pg_xlog
5. File 00000002000000000000000A in archive
6. File 00000002000000000000000A in pg_xlog

With this patch, the order is:

1. File 00000004000000000000000A in archive
2. File 00000003000000000000000A in archive
3. File 00000002000000000000000A in archive
4. File 00000004000000000000000A in pg_xlog
5. File 00000003000000000000000A in pg_xlog
6. File 00000002000000000000000A in pg_xlog

This change should have no effect in normal restore scenarios. It'd only 
make a difference if some files in the middle of the sequence of WAL 
files are missing from the archive, but have been copied to pg_xlog 
manually, and only if that file contains a timeline switch. Even then, I 
think I like the new order better; it's easier to explain if nothing else.

- Heikki