WIP: add metadata to dump output

[David Bremner]

It seems (at least to me) that xapian metadata is the right way store
certain configuration data, including tag aliases [1] and perhaps some
non-CLI specific configuration. On the other hand we don't want to
have things lost if we dump and restore a database. Hence this series,
which is a start at dumping and restore such config.

The main idea here is that various classes of metadata can be defined
by using prefixes, in exactly the same way as tags are defined for
documents. This will hopefully help prevent e.g. config from stomping
on tag aliases.

The first 6 patches impliment iterators for simple "queries" on
metadata. They are probably split a bit fine, but that's the way I
developed them.

The last 3 impliment the printing of metadata in dump output. In order
to be upwardly compatible, it uses the old dodge of hiding things in
comments. In fact the comment syntax (# in first column) was never
well documented; this does mean that the notmuch dump output can be
tested without breaking the current restore tests. I threw an @ in to
help autodetection of formats; obviously this is not foolproof. On the
other hand, I don't know how much people currently rely on comments in
dump files, since notmuch doesn't generate them.

There's lots of bikes to shed here. Probably the most important bits
are the library API, the dump output format, and of course the ever
tricky command line argument names.

Getting the memory ownership semantics is tricky, especially with the
mix of C++ objects and talloc. So I'd appreciate a critical eye on
those bits of metadata.cc.

[1]: id:1448504191-30974-1-git-send-email-igor.contato at gmail.com