Cloud Spanner Client should prevent nested transactions

[vkedia]

Cloud Spanner does not support nested transactions. But the client library does allow users to call Database.runTransaction inside of the callback that is provided to an outer Database.runTransaction. This is misleading since users might believe that this will behave like a nested transaction but in reality these will be two independent transactions. This is confusing and a source of bugs. Specifically the inner transaction might succeed while the outer might ABORT, in which case the callback will be rerun thus rerunning the inner transaction.
We should prevent this from happening by detecting and raising an error if someone calls Database.runTransaction inside the callback provided to Database.runTransaction. Note that this needs to be done before Beta since this is a known breaking change.
Also note that if we come across a use case where we do want to allow calling a nested runTransaction, we should be able to enable that in future without making a breaking change.

cc @jgeewax @bjwatson

[michaelbausor]

GA-blocking issues shouldn't be marked P0; P0 is for issues where the latest release is DOA. Downgrading to P1. (As per googleapis/google-cloud-java#2117)

[vkedia]

For PHP this is Beta blocking since this is a breaking change in the interface.

[michaelbausor]

Yes, sorry. I didn't mean to imply that this was not a beta blocking issue - I should have said release-blocking rather than GA-blocking.

[bjwatson]

👍