Drop support for pre-1.4 Xapian, prepare for 1.5.x

[David Bremner]

The current development release of Xapian drops several deprecated
APIs. This hampers testing of notmuch against Xapian master (i.e. to
see if a problem has been fixed). This series fixes most of the
incompatibilities.

The one thing this series doesn't handle is test/T530-upgrade.sh,
which mostly fails because Xapian 1.5 / master drops support for
Chert. I think that we should probably deprecate support for notmuch
v1 databases. These were replaced by version 2 in 2010. I think more
thought is probably needed for this migration so I left those failing
tests for now.  This isn't quite as scary as it sounds, because we
really only support stable releases of Xapian, so the chert removal
effectively won't happen until 1.6.