Support v2

[rpt]

There's v2 version of the protocol. Implement it when it's stable.

[rpt]

Changes from v1:

9. Changes from v1
  * Protocol is versioned to allow old client connects to a newer server, if a
    newer client connects to an older server, it needs to check if it gets a
    ProtocolException on connection and try connecting with a lower version.
  * A query can now have bind variables even though the statement is not
    prepared; see Section 4.1.4.
  * A new BATCH message allows to batch a set of queries (prepared or not); see 
    Section 4.1.7.
  * Authentication now uses SASL. Concretely, the CREDENTIALS message has been
    removed and replaced by a server/client challenges/responses exchanges (done
    through the new AUTH_RESPONSE/AUTH_CHALLENGE messages). See Section 4.2.3 for
    details.
  * Query paging has been added (Section 7): QUERY and EXECUTE message have an
    additional <result_page_size> [int] and <paging_state> [bytes], and
    the Rows kind of RESULT message has an additional flag and <paging_state> 
    value. Note that paging is optional, and a client that do not want to handle
    can simply avoid including the Page_size flag and parameter in QUERY and
    EXECUTE.
  * QUERY and EXECUTE statements can request for the metadata to be skipped in
    the result set returned (for efficiency reasons) if said metadata are known
    in advance. Furthermore, the result to a PREPARE (section 4.2.5.4) now
    includes the metadata for the result of executing the statement just
    prepared (though those metadata will be empty for non SELECT statements).