Warn if PreparedStatement's consistency level is ignored

[nyh]

Our documentation of PreparedStatement (see https://java-driver.docs.scylladb.com/stable/manual/core/statements/prepared/) doesn't explain too clearly what the ConsistencyLevel options does (Datastax's documentation, https://docs.datastax.com/en/drivers/java/2.1/com/datastax/driver/core/PreparedStatement.html, is clearer), but to make a long story short:

The consistency level given when preparing a PreparedStatement has no immediate effect. All that happens is that the given consistency-level value is saved in the PreparedStatement Java object - and only later when the prepared statement is bound into a BoundStatement, this saved consistency-level is inherited by the new object, and when the bound statement it is executed, the consistency level is sent via the CQL protocol. So far so good.

@michoecho discovered a case where a user was using a batch statement with a prepared statement in it, something like

BatchStatementBuilder batchStatementBuilder = new BatchStatementBuilder(BatchType.LOGGED);
batchStatementBuilder.addStatement(myPreparedStatement.bind(...));

The problem is that a batch statement has its own consistency level setting, and only that setting is sent to the server (CQL only allows one consistency setting per statement - even if it's a batch). The consistency level variable set in myPreparedStatement and copied to the bind() output is outright ignored!

Note how confusing this is - the user may think that he created a prepared statement with QUORUM consistency, but when when inserting it into a batch statement, it ends up executed using the batch statement's default consistency level which is usually ONE, resulting in incorrect consistency!

I think BatchStatementBuilder should check if any of the of the statements added by addStatement() has a different (or just higher?) consistency level requirement than it itself has, and if that situation is detected, the request should produce a run-time warning, or even run-time error and not run at all.

I don't know if we have a similar problem also in drivers of other languages :-(