Fix downward migrations / allow initialization without both ClientSettings if ConnectionString and DatabaseName have been provided

[ssteiner]

This PR solves two issues:

1. When downgrading the database version by setting MongoMigrationSettings.DatabaseMigrationVersion, the DatabaseMigrationRunner would run not only migrations with a version that is smaller than currentVersion, but any version that is not equal to toVersion.

2. On .Net core, when you're providing a ConnectionString and Database in the MongoMigrationSettings, initialization would fail because of two checks that MongoMigrationSettings incorrectly enforce that both MongoMigrationSettings.ConnectionString and MongoMigrationSettings.ClientSettings be populated. Subsequent code shows that this isn't what was intended - initialization was prepared for both cases: when there's only a ConnectionString and when there's only ClientSettings . Hence I changed the initialization checks accordingly to allow initialization if only one of ConnectionString / ClientSettings is provided in MongoMigrationSettings.