Load Balancer Support

Summary: This is Swift's implementation of load balancer. Engineer(s): Kaitlin Mahar 2021-09-03: Setting target date to 2021-09-10 Final ticket is in review, pending Patrick's return from PTO. Expecting it will be merged in by Wednesday at the latest but setting target date to the next Friday checkpoint. Effort is essentially complete! Engineer(s): Kaitlin Mahar 2021-08-25: Setting target date to 2021-09-01 Adding a few days to account for Kaitlin (implementer) and Patrick (reviewer) taking PTO, as well as some unexpected extra test runner changes that had to be made. Adding 1 week to cost since Kaitlin has been spending max ~50% of time on this. All public-facing API changes are done, and all work to get existing tests running/passing against a local load balancer is complete. Remaining work is to get new load balancer-specific tests running, and modify Evergreen config to test against load balancers on there. Engineer(s): Kaitlin Mahar 2021-08-11: Setting target date to 2021-08-27 Kaitlin has begun picking up this work and is making progress when she is not focusing on management duties. So far directConnection=false has been enabled in loadBalanced=true URIs, and support for the loadBalanced option in the connection string and MongoClientOptions is currently under review.