chore(spanner): add R/W multiplexed session support

[harshachinta]

This PR introduces support for using multiplexed sessions in R/W transactions for the following methods:

1. write
2. writeWithOptions
3. readWriteTransaction
4. transactionManager
5. runAsync
6. transactionManagerAsync

[olavloite]

Same here as above: Preferably, these methods should be non-blocking if possible.

[harshachinta]

Modifying this to be non-blocking would require the method signature to be modified, which makes it impossible here. Should we just accept this as it is which means this will be blocking until a multiplexed session is available.

Please let me know if there are any other ways to handle this?

[olavloite]

I'm not sure I understand why the signature would need to change. The return value is a TransactionContextFuture (so something that extends Future). That being said; If it requires a lot of work or refactoring, then it is certainly not worth it, so I'm fine with leaving this as is.

[harshachinta]

We might need to use ApiFutures.transformAsync to make beginAsync() to be non-blocking. However, the return type of transacformAsync is a ApiFuture<O> which does not conform to the return type of this method which is TransactionContextFuture. So I am assuming the signature need to be changed to ApiFuture<TransactionContextFuture>.

However, since TransactionContextFuture extends Future, just typecasting could be sufficient.
That said, let me pick up this analysis in a seperate PR to get the rest of PRs unblocking.

[olavloite]

I'm not sure I understand why the signature would need to change. The return value is a TransactionContextFuture (so something that extends Future). That being said; If it requires a lot of work or refactoring, then it is certainly not worth it, so I'm fine with leaving this as is.