TrigFastTrackFinder: optimize displaced vertex hit storage

CPU optimizations for displaced vertex hit storage:

• pre-populate DataVector instead of using push_back
• use Accessor instead of setDetail (2% speedup by itself)

Results in a total speedup of about 13% in TrigFastTrackFinder_jet where this feature is used by default.

Relates to ATR-27846. cc @smh @sutt