Allow named parameters in simple statements

We're going to need the ability to send simple statements with named parameters for upcoming DSE features.

We did a first attempt with JAVA-855, but went a bit too far with parameter getters/setters. We just need a new constructor SimpleStatement(String query, Map<String, Object> values), a new method on RegularStatement that will return the names, and the internal changes for the protocol.

Resolution: new constructor SimpleStatement(String, Map<String, Object>), and corresponding execute and executeAsync methods on Session