Update spanner emulator version

[scotthart]

We're currently using v1.5.16 of the spanner emulator which does not have full support for multiplexed sessions. Attempting to update to the latest spanner emulator v1.5.28 causes new failures in existing tests meaning some behavior has changed across emulator versions, that may or may not indicate real errors.

We need to update the emulator to the latest version and update existing tests (and or library code).

[devbww]

It looks like 1.5.28 must have gained support for Database.version_retention_period, although without the check for zero being invalid.

In my day, the spanner-emulator folks ran our integration tests as part of their validation process. I guess they either don't do that any longer, or worked around these failures without warning us.