Since we are already passing a search context around as a kind of
parameter block, add a new talloc context to that to replace relying
on 'config'.
Convert notmuch-search and notmuch-address at the same time, because
they share some code.
Add a test to make sure we don't break passing configuration as a
command line argument.
The new talloc context is needed to run the hook at the very end of
the function. That in turn is needed so that this process gives up the
write lock on the database.
This conversion is trivial because the only configuration information
accessed by dump is that stored in the database (in order to dump
it). We do need to be careful to keep the write lock on the database
to ensure dump consistency.
The main effort is changing from the old argv style config list
iterators to the new more opaque ones provided by the library (and
backed by the database+file config cache).
