Check for existence of database

[ThomWright]

createDb attempts to create a database. If it already exists, it detects the 'duplicate' error and resolves appropriately.

However, on a readonly replica, it gets a different error.

Now, arguably migrations shouldn't be running on a replica... but let's move past that for now. If there are no new migrations to run then I think this library should be happy being run against a read replica.

So, what we could do is:

• create a connection, then on that connection:
  • check if the database exists
  • if not, then try to create
• close the connection

It looks like SELECT 1 FROM pg_database WHERE datname='DB_NAME' might do it.

I think the API could probably do with changing too... it is more ensureDbExists than createDb. Would have to keep createDb around for backwards compatiblity.