DRIVERS-2782 Allow variable access to snapshotTimeout

[prestonvasquez]

DRIVERS-2782

Please complete the following before merging:

• Is the relevant DRIVERS ticket in the PR title?

Go Driver POC: mongodb/mongo-go-driver#2271

• Update changelog.
• Test changes in at least one language driver.
• Test these changes against all server versions and topologies (including standalone, replica set, and sharded
  clusters).

The Go Driver sets snapshotTime an experimental API:

type ClientSession{
  Snapshot *bson.Timestamp // Pointer reference
}

Users could easily update a session and use it in the pathological case

sess.ClientSession().Snapshot = &bson.Timestamp{T:1, I:1}
_ = sess.StartTransaction(options.Transaction().SetReadConcern(readconcern.New(
		readconcern.Level(readconcern.Snapshot().Level),
		
)))

_, _ = coll2.InsertOne(mongo.NewSessionContext(context.Background(), sess), bson.D{})

err = txnSes.CommitTransaction(context.Background())
// fails with: "(WriteConflict) Caused by ::  :: Please retry your operation or multi-document transaction."
// mongod logs show "Conflict with update with txn id [...] at timestamp: ([T2])"
require.NoError(t, err)

Drivers should ensure that snapshotTime is immutable, if it's already publicly readable. If it's not immutable, the behavior should be considered a bug.

[papafe]

LGTM

[sanych-sun]

Doesn't this statement conflicts with requirement of immutability?

The snapshotTime field MUST be immutable
A readonly property called snapshotTime will be added to ClientSession that allows applications to retrieve the
snapshot time of the session:

Why would we have any public API to mutate the value? Or we are talking about a languages with no technical ability to make the field readonly?

[sanych-sun]

As far as I can say this is not a 2 approaches, this is simply 2 requirements:

1. snapshot time should throw if snapshot = false (or return some special "Invalid" value if throwing is not an option).
2. snapshot MUST be immutable.