SDK should reject malformed connection strings

Prior to Java SDK 3.4.1, the SDK accepted malformed connection strings that had repeated schemes, like this:

couchbase://foo.example.com,couchbase://bar.example.com

In a valid connection string, the scheme prefix ("couchbase://" in this case) may appear at most once. The correct way to include multiple hosts is:

couchbase://foo.example.com,bar.example.com