Use seastar::checked_ptr<weak_ptr<pepared_statement>> instead of shared_ptr for passing prepared statements around.
This allows an easy tracking and handling of statements invalidation.
This implementation will throw an exception every time an invalidated
statement reference is dereferenced.
Signed-off-by: Vlad Zolotarov <vladz@scylladb.com>
To prepare for the separation of prepared and raw schema_altering_statements,
avoid the reliance this class implementing both the raw and prepared
variants and the use of shared_from_this().
