CDRIVER-4718 store recoveryToken for Load Balanced topology

[kevinAlbs]

Summary

• Store recoveryToken for Load Balanced topology

Background & Motivation

Bug was discovered running proposed specification test changes with the PHP driver: mongodb/specifications#1459 (comment)

Additionally, several other pinning tests fail due to recoveryToken fields not being found in outgoing commitTransaction and abortTransaction commands

The Load Balancer specification notes:

When executing a transaction in load balancing mode, drivers MUST follow the rules outlined in Sharded Transactions

Sharded Transactions describes expected behavior for recoveryToken:

Drivers MUST track the most recently received recoveryToken field and MUST append this field to any subsequent commitTransaction or abortTransaction commands.

Aside: Connection pinning

The Load Balancer specification describes required behavior for pinning all transaction operations to one connection:

The rules for pinning to a connection and releasing a pinned connection are the same as those for server pinning in non-load balanced sharded transactions

This is intentionally not implemented in libmongoc due to not being needed. CDRIVER-4056 notes:

Not in scope
Connection pinning to cursors or transactions. Rationale: The C driver does not implement a CMAP compliant connection pool (see CDRIVER-2871). Only one connection is available to a server for all operations within a client.

This motivated keeping a separate _in_sharded_txn (to check for mongos pinning) from _in_sharded_or_loadbalanced_txn (to check for recoveryToken)