Add support for Google Cloud Spanner

[olavloite]

This adds support for Google Cloud Spanner (https://cloud.google.com/spanner) to Flyway. The integration relies on the open source JDBC driver for Google Cloud Spanner (https://github.com/olavloite/spanner-jdbc) instead of the one provided by Google, as the official driver is read only.

Context

The DDL syntax and possibilities of Cloud Spanner are close to standard SQL, albeit rather limited. Further specifics:

• Cloud Spanner does not support schemas. All databases contain a default schema named '' (emtpy string) and a system schema named INFORMATION_SCHEMA. The default schema may not be explicitly referenced, i.e. the statement CREATE TABLE ``.TEST (ID INT64 NOT NULL, ...) PRIMARY KEY (ID); will fail.
• Cloud Spanner does not support foreign keys, but tables can be interleaved in each other. When dropping interleaved tables, the child table must be dropped before the parent table.
• Tables can only be dropped after all secondary indices on the table have been dropped.
• DDL statements are very slow, especially if executed sequentially. The JDBC driver therefore supports both (automatic) batching and asynchronous execution of DDL statements.

Example Application

I have created an example application showing the working of Flyway with Google Cloud Spanner. Google Cloud Spanner is only available in the Google Cloud, but you can opt for a free trial account a $300 credit for testing it.
The application can be found here: https://github.com/olavloite/spanner-flyway-example
An article describing the example can be found here: http://www.googlecloudspanner.com/2018/01/google-cloud-spanner-with-flyway.html

[CLAassistant]

All committers have signed the CLA.

[tuanng-cognite]

any plan to merge this?

[luiccn]

Any plans on merging this? Very interested in using this the proper way :)

[thinhha]

Ping @olavloite any plans to merge this please? :)

[DoodleBobBuffPants]

Thanks for the PR! This is now available in the V7 beta which you can read more about here.