searchidx: rename get_val to int_val and return IV
Values can be strings in Xapian, although we currently use
integer values exclusively. Give the wrapper a more appropriate
name in case we start using string columns.
For future-proofing, we'll now return `undef' on missing columns
and coerce the return value to an IV (integer value) to save
memory, as sortable_unserialise returns a PV (pointer value)
scalar despite it existing to support numeric values.