Support customization of spanner host/port

[elefeint]

When Cloud Spanner emulator is available, users will need a way to specify the local connection host/port.

SpannerConnectionConfiguration is a natural place for it -- right now it parses project/instance/database information out of a fully qualified connection string, but it could as easily get host/port information to be passed to GrpcClient.
We'd also need a couple of configurable options for this, and GrpcClient needs to stop treating them as constants.

[elefeint]

This is likely obsolete due to V2 redesign -- V2 is based on the client library, which acquires emulator host/port based on environment variable.

But this behavior needs to be verified.

[elefeint]

Will re-test.

[elefeint]

Verified that simply testing SPANNER_EMULATOR_HOST=localhost:9010 is sufficient to run R2DBC v2 driver against the emulator.
In the process, found a small issue with bindNull() TCK test re-implementation -- it was starting a transaction but not committing it (the superclass bindNull() is meant to run in autocommit mode, so it does not need to start a new transaction at all, so it's a problem with our override). But this is moot, since this test override will go away after #270 is merged.

But! Even though the test issue is moot, it exposed what I thought was a problem with not cleaning up the transaction manager in the driver (#279), but is likely a missed bit of clean up code in the new asynchronous transaction manager in the client library (googleapis/java-spanner#504).

I am also thinking that even though the environment variable unblocks the use of emulator, we should still provide ability to customize host/port through connection properties (both connection URL string like the JDBC driver does and the programmatically constracted SpannerConnectionConfiguration. To do this, customize SpannerClientLibraryConnectionFactory constructor to set SpannerOptions.newBuilder().setEmulatorHostPort().