cassandra.cql: Use a dict of statements instead of dynamically building method names

in the two methods which need to switch between statements.

The method name building was done to shoehorn this statement switching into the existing @_prepared_select_statement.

This introduces @_prepared_select_statements (plural), which does this switching properly, using a dictionary.

Depends on !532 (closed).

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Migrated from D3787 (view on Phabricator)