Collect information about prepared statements if P_S is enabled

In version 5.7+ it is possible to see statistics of all prepared statements, currently existing in the server:

select t.processlist_id, pse.* from performance_schema.prepared_statements_instances pse join performance_schema.threads t on (pse.OWNER_THREAD_ID=t.thread_id);

Launchpad: https://bugs.launchpad.net/percona-toolkit/+bug/1642750