Enable event-local memory pool in PV reconstruction and selection algorithms

Depends on LHCb!2343 (merged), makes sense to test with Phys!650 (merged).

Changes the transient type for PVs to be LHCb::Event::v2::RecVertices.

Also:

• Propagates allocators through PrFilterIPSoA.cpp
• Enables the event-local memory pool for vertex relations.
• Enables the event-local memory pool for CombineTracks and adds a missing reserve().
• Enables the event-local memory pool for both TrackBeamLineVertexFinderSoA both for its output and intermediate objects.

Note that CombineTracks will not compile without gaudi/Gaudi!1017 (closed) or gaudi/Gaudi!1031 (merged).

cc: @nnolte @wouter @ahennequ