This commit removes the geoNear command and moves its implementation
into the aggregation framework. Users should use the aggregate command
with a $geoNear stage.
The implementation rewrite additionally removes the limit in the
$geoNear aggregation stage. To limit the number of results, use a $limit
stage.
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.
