Make DeleteScheduler tests more reliable

Make DeleteScheduler tests more reliable

Summary: Update DeleteScheduler tests so that they verify the used penalties for waiting instead of measuring the time spent which is not reliable

Test Plan:
make -j64 delete_scheduler_test && ./delete_scheduler_test
COMPILE_WITH_TSAN=1 make -j64 delete_scheduler_test && ./delete_scheduler_test
COMPILE_WITH_ASAN=1 make -j64 delete_scheduler_test && ./delete_scheduler_test

make -j64 db_test && ./db_test --gtest_filter="DBTest.RateLimitedDelete:DBTest.DeleteSchedulerMultipleDBPaths"
COMPILE_WITH_TSAN=1 make -j64 db_test && ./db_test --gtest_filter="DBTest.RateLimitedDelete:DBTest.DeleteSchedulerMultipleDBPaths"
COMPILE_WITH_ASAN=1 make -j64 db_test && ./db_test --gtest_filter="DBTest.RateLimitedDelete:DBTest.DeleteSchedulerMultipleDBPaths"

Reviewers: yhchiang, sdong

Reviewed By: sdong

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D43635