Improve performance scaling with host threads

[esseivaju]

Improve performance scaling with host threads

[sethrj]

@esseivaju I think you've done a great job addressing this. Could you please add links to the relevant PRs, take a new snapshot of the timing, and close this out?

[esseivaju]

Comparison of scaling with host threads between Celeritas v0.3.2 and 2332351. Measured on Perlmutter using the Atlas Tilecal example, vecGeom 1.2.5 and Geant4 11.0.1. Relevant PRs:

• #908
• #910
• #953
• #965 (as of this comment, only helps if track sorting is enabled, which was turned off in this benchmark)

As mentioned in the first comment, the only async copy to pageable memory remaining are return values from thrust calls. There is no way to change that (see this discussion) and the gains would most likely be small since we have to synchronize each stream at the end of each step anyway so it's not a top priority to fix.