Introduces OverrideHelpers object with convenience methods for
inspecting certain aggregation and map-reduce commands, as well as
overriding startParallelShell(), Mongo.prototype.runCommand(), and
Mongo.prototype.runCommandWithMetadata().
Also removes a number of tests that were incorrectly blacklisted from
the read_concern_majority_passthrough.yml and
read_concern_linearizable_passthrough.yml test suites.
In addition to improving test coverage for document sequences, this also
improves performance of insert commands sent over OP_QUERY since they will no
longer copy the objects during upconversion.
