Scaling in traccc with vecmem v.1.23 with CUDA event caching

Much better scaling as-is.

L40

H100

• cudaLaunchHostFunc seems costly for suspension synchronization. Events investigated as a replacement.
• Delegation does help mitigate the suspension cost but TBB seems to hang in some combinations of threads/slots. Alternative to investigate: delegate to a bare thread instead of a TBB 1-thread arena.