Merge custom memory pool changes

As requested by @rmatev on !2270 (merged).

The complete set of MRs that relate to custom memory pools are:

• gaudi/Gaudi!1026 (merged) (no dependencies; some duplication can be removed once gaudi/Gaudi!1026 (merged) and !2270 (merged) are both merged)
• !2270 (merged) (adds infrastructure, no dependencies)
  • !2338 (merged) (depends on !2270 (merged), may need to wait for a ROOT patch)
  • !2339 (merged) (depends on !2270 (merged))
    • Rec!1892 (merged) (depends on !2339 (merged))
  • !2340 (merged) (depends on !2270 (merged))
    • Rec!1893 (merged) (depends on !2340 (merged))
  • !2341 (merged) and Rec!1895 (merged) (depend on each other and on !2270 (merged))
  • !2342 (merged) (depends on !2270 (merged))
  • !2343 (merged) (depends on !2270 (merged))
    • Rec!1896 (merged) (depends on !2343 (merged))
      • Phys!650 (merged) (depends on Rec!1896 (merged))

I tried to tabulate the actual dependencies that need to be respected, but I think this makes it look much more complicated than we really need to worry about. I would suggest:

• Ignore gaudi/Gaudi!1026 (merged), which can be handled asynchronously
• Merge !2270 (merged) "now"
• Group the rest (excluding !2338 (merged)) as Enable event-local memory pool for LHCb::Pr::Ve... (!2339 - merged)Rec!1892 (merged), Enable event-local memory pool for LHCb::Pr::{U... (!2340 - merged)Rec!1893 (merged), Make classes relevant to the SciFi/forward trac... (!2341 - merged)Rec!1895 (merged), !2342 (merged), Enable event-local memory pool for vertex and s... (!2343 - merged)Enable event-local memory pool in PV reconstruc... (Rec!1896 - merged)Phys!650 (merged). It should be fine to do anything on the spectrum from testing+merging each group separately to testing+merging all 5 groups in one go. This can just be decided based on their review/approval status at the relevant time.
• Merge !2338 (merged) "later" once the ROOT fix has been verified and propagated to LCG etc.