Issue Description

We have a plugable index subsystem, however we don't use it anywhere in the code base. In the past it provided the ability to assign functions to filters to allow out-of-band indexes. However now all it provides was a filter-fold mechanism (which the new filter optimiser replaces).

Since we have no users of this subsystem, we should deprecate and remove this. I do not foresee us accepting pluggable indexes again, as there is little community or commercial interest.