Adds a mock server and several end-to-end tests using the mock server.
Also fixes:
- Invalid function calls in migrations
- Unnecessary use of snapshots for INFORMATION_SCHEMA queries
- Unnecessary SELECT 1 query each time a connection is checked out of the pool

Enables running the acceptance integration tests on the emulator. These finish in ~20 seconds
instead of after several hours. All tests are green, except for one that is skipped, as
Cloud Spanner does not support changing a nullable column to a not-null column for a table
that already exists.

Updates #65

Enables running the acceptance integration tests on the emulator. These finish in ~20 seconds
instead of after several hours. All tests are green, except for one that is skipped, as
Cloud Spanner does not support changing a nullable column to a not-null column for a table
that already exists.

Updates #65

…loite/ruby-spanner-activerecord into enable-acceptance-tests-on-emulator