feat(NODE-3470): retry selects another mongos

[durran]

Description

When retrying reads and writes on a sharded cluster, will attempt the retry on a different mongos if multiple mongoses are present.

What is changing?

• executeOperation now keeps track of the previously selected server.
• Server selection will now attempt to exclude the previous server when the topology is sharded and multiple mongoses are present.
• Adds unit tests for server selection and prose tests for retryable reads and writes.

Is there new documentation needed for these changes?

None

What is the motivation for this change?

NODE-3470

Release Highlight

When retrying reads or writes on a sharded cluster, the driver will attempt to select a different mongos for the retry if multiple are present.

Double check the following

• Ran npm run check:lint script
• Self-review completed using the steps outlined here
• PR title follows the correct format: type(NODE-xxxx)[!]: description
  • Example: feat(NODE-1234)!: rewriting everything in coffeescript
• Changes are covered by tests
• New TODOs have a related JIRA ticket