Parallelize Invocations::countUsedFingerprints
Parallelize Invocations::countUsedFingerprints

This saves around 10-15ms on my largish test project when parsing
the invocation log. I am extra interested in this because I want to
reuse this logic in build.cpp later on, to parallelize the cleanliness
checks.

per-gron/shuriken,per-gron/shuriken,per-gron/shuriken,per-gron/shuriken