First attempt at implementing changeset dumps.

Changeset-only dumps are pretty useful for doing some analysis
which doesn't require downloading the whole planet, and the old
planet dump script supported it, so it seems like a good idea to
try and support it here too.

This adds a "filter" applied to `xml_writer` which drops all
elements except for changesets, which is pretty much what we want.
Hopefully the overhead of the extra synchronisation with the dump
thread won't be too high.
Merge remote-tracking branch 'origin/master' into changesets

Conflicts:
	src/Makefile.am
	src/planet-dump.cpp