lib: migrate from Xapian ValueRangeProcessor to RangeProcessor

This will be mandatory as of Xapian 1.5.  The API is also more
consistent with the FieldProcessor API, which helps code re-use a bit.

Note that this switches to using the built-in Xapian support for
prefixes on ranges (i.e. deleted code at beginning of
ParseTimeRangeProcessor::operator(), added prefix to constructor).

Another side effect of the migration is that we are generating smaller
queries, using one OP_VALUE_RANGE instead of an AND of two OP_VALUE_*
queries.