chore(spanner): preserving lock order - R/W mux session

[harshachinta]

This PR introduces changes to support the lock order preservation protocol for multiplexed sessions in read/write transactions. According to this protocol, when a read/write transaction on a multiplexed session is retried, the transaction ID from the previous abort must be passed when creating a new transaction. This ensures that the retried transaction is recognized as older, rather than being treated as a new one.

The transaction context object is structured as follows,

txn {
    transactionId: The transaction ID.
    previousTransactionId: The transaction ID of the most recently failed transaction.
}

[harshachinta]

Test Scenario

1. Transaction 1 is started and the inline begin fails due to invalid update statement. This leads to ABORTED exception and transaction is retried.
2. Transaction 2 is initiated with explicit begin transaction which succeeds and creates a transactionId (txn2). however transaction2 gets aborted during commit (The test forces the commit rpc to return ABORTED the first time it is called).
3. Transaction 3 is started but again inline begin fails. Here txn id is null, but prevTransactionId will be txn2.
4. Transaction 4 is started with explicit begin. As Transaction 3 never got an ID, the transaction id of Transaction 2 (txn2) is used in TransactionOptions.

This behavior is verified in test by checking that second explicit BeginTransaction RPC carries the txn2 as previous transaction id.

[olavloite]

Looks generally good to me. Waiting with approving until we can run the tests and verify that those are all green.

[harshachinta]

Looks generally good to me. Waiting with approving until we can run the tests and verify that those are all green.

@olavloite The checks are green. Can you please help review?

[olavloite]

LGTM, but with a minor request on making previousTransactionId final.