feat(cql) support for native protocol v4

handle frame responses warnings. res result can now contain a warnings array. The resty.cassandra.cluster module logs those as warnings.
new null type serializer (cassandra.null shorthand)
provide FUNCTION and AGGREGATE info in SCHEMA_CHANGE results
fix an issue in the cluster module when connecting to nodes not supporting the latest protocol version (that is, v4 as of this PR)
add documentation to cql.lua as to the supported native protocol versions, and what't not implemented

thibaultcha / lua-cassandra