Statement restart

Implement statement restart inside MariaDB Server query processor with a designation mechanism driven by storage engine (handler).

The rationale is mainly an optimization of discovery process for remote engines.

During the table open, MariaDB requests Backoff_processor from handler.
Later, during the query run, after an error happens, Backoff_processor analyzes the error for a matter of backoff.
In do_command, in case of error+backoff, the last statement is re-run instead of fetching the new statement.

New functions/entities:

• handler::get_backoff_processor() - returns Backoff_processor object
• emum_backoff_aciton { BA_NO_ACTION, BA_BACKOFF_AND_RETRY };
• Backoff_processor class has a virtual function decide_backoff_action(int error), returning enum_backoff_action