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