tracing: Do not start unnecessary threads.
tracing: Do not start unnecessary threads.

MapRunner always starts a lot of threads (= # of cores), even when it
only needs to process a single trace. Update the code to create no more
threads than the number of traces it needs to process.

BUG=chromium:942288

Change-Id: I50ee4cfdc2162ac8eb708ba5366a3f9cc3d4369a
Reviewed-on: https://chromium-review.googlesource.com/c/catapult/+/1531738
Auto-Submit: Sadrul Chowdhury <be235e35a6c144329e4aa1b0fbe2c3f8c71f3676@chromium.org>
Commit-Queue: Caleb Rouleau <58bdb6276c01e3ed53e660bf706aa902204feb0d@chromium.org>
Reviewed-by: Caleb Rouleau <58bdb6276c01e3ed53e660bf706aa902204feb0d@chromium.org>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult