Enable event-local memory pool for LHCb::Pr::{UT::Hits,Upstream::Tracks}
Uses the custom allocator of !2270 (merged) in LHCb::Pr::Velo::{Hits,Tracks}.
Targets the branch of !2270 (merged) so the diff is correct. Hopefully we can merge that one imminently; for now this makes the diff appear correctly.
Without Rec!1893 (merged) the behaviour is unchanged (i.e. a default-constructed container uses the system allocator), but it's probably most efficient to test this in conjunction with that MR.