Acceptance Criteria

• Pull in all the new spec tests and check for failures.
• Implement the base snapshot sessions functionality according to the spec
  • Introduce `snapshot` session option for use in `client.startSession` (optional, default false)
  • Driver should raise error if `snapshot` and `causalConsistency` are both set to `true`
    • Should add unit test
  • Driver should raise error if `snapshot` is `true` and `session.startTransaction` is invoked
    • Should add unit test
  • ReadConcernLevel enum updated to include "snapshot"
    • NOTE: this already exists in the driver
  • Implement snapshot functionality for the following commands: `find`, `aggregate`, `distinct`
    1. Specify read concern `level: 'snapshot'` with the command; save the `atClusterTime` returned by 5.0+ servers for the first find/aggregate/distinct operation in a private property `snapshotTime` of the `ClientSession` object
    2. Pass that `snapshotTime` in the `atClusterTime` field of the `readConcern` field
       for subsequent snapshot read operations (for find/aggregate/distinct commands).
• Newly added spec tests should pass

Not required for this ticket:

• Raising a client-side error when attempting to use a snapshot session with server versions < 5.0 (NODE-3394)
• Sending readconcern with `snapshot` and `atCluserTime` properties on all snapshot session operations, even writes (NODE-3393)