RUBY-1250 Run DriverBench benchmarks in Evergreen

[jamis]

This is a large PR, primarily new files, though most of the new files are small.

The main files to consider are:

• DriverBench::Suite in profile/driver_bench/suite.rb
• DriverBench::Base in profile/driver_bench/base.rb

The benchmarks are run via DriverBench::Suite, which accumulates the results, computes the scores, and ultimately emits the results as a JSON file. The JSON file is then sent to the evergreen performance plugin via perf.send.

Each benchmark is ultimately a subclass of DriverBench::Base. The core of the benchmark is implemented in the do_task method, which DriverBench::Base#run_benchmark invokes. Other lifecycle hooks (setup, before_task, after_task, and teardown) are used as described in the specification, to manage the state of the benchmark.