Drop support for pre-1.4 Xapian, prepare for 1.5.x (v2)

[David Bremner]

This obsoletes id:20190414124445.3394-1-david at tethera.net

Compared to the previous version it

    - is rebased against master
    - has some unfortunate whitespace changes to INSTALL reverted.
    - now includes a removal of T530-upgrade.sh and supporting machinery.

I'm not sure about the last point. It is potentially possible to
regenerate a v1 database with a non-chert xapian backend, but I'm not
sure it's worth the trouble. I did notice that devel/gen-testdb.sh is
currently broken due to changes in the test framework.