Refactor pg_rewind code and make it work against a standby

Hi,

I started to hack on making pg_rewind crash-safe (see [1]), but I
quickly got side-tracked into refactoring and tidying up up the code in
general. I ended up with this series of patches:

The first four patches are just refactoring that make the code and the
logic more readable. Tom Lane commented about the messy comments earlier
(see [2]), and I hope these patches will alleviate that confusion. See
commit messages for details.

The last patch refactors the logic in libpq_fetch.c, so that it no
longer uses a temporary table in the source system. That allows using a
hot standby server as the pg_rewind source.

This doesn't do anything about pg_rewind's crash-safety yet, but I'll
try work on that after these patches.

[1]
https://www.postgresql.org/message-id/d8dcc760-8780-5084-f066-6d663801d2e2%40iki.fi

[2] https://www.postgresql.org/message-id/30255.1522711675%40sss.pgh.pa.us

- Heikki