feature(spanner): Implement DML Support for Spanner

[jtwatson]

Closes #135
Closes #775
Closes #918
Closes #1194

Supersedes the following PRs #733, #910, #924

1. This PR implements DML support and uses the memefish lexer to parse the migrations, replacing the usage of the spansql package. As a result, numerous other spansql related issues have been resolved.

   The following features have been implemented:

   1. DML support
   2. Ability to put multiple statements in a single migration script
   3. Ability to include comments in the migration scripts
   4. Ability to mix DDL and DML in the same migration script

   To maintain backward compatibility, the x-clean-statements option must be used to enable the above features.

2. This PR also fixes a bug where a provided spanner client via WithInstance() is closed when Migration.Close() is called. According to Docs, the caller is responsible for closing the underlying database client. This fix brings the Spanner driver into compliance with the expected behavior.

I am maintaining a fork with this fix for those who would like to use it.

Put the following replace directive you your projects go.mod

replace github.com/golang-migrate/migrate/v4 v4.18.1 => github.com/jtwatson/migrate/v4 v4.18.2-beta.2

[jtwatson]

@dhui

This PR supersedes the following PR #733, #910, #924

It is a robust implementation that greatly improves support for Spanner migrations. It also fixes a lot of open issues with the existing DDL parsing implementation.

NOTE: Not sure why tests are failing, but it is not related to spanner tests. It looks like cockroachdb is having an issue.