…_history_size

Accept a reference to a vector instead of returning a deque

MySQL_Logger::insertMysqlEventsIntoDb() populate table `stats_mysql_query_events`
using mysql query events in the circular buffer

MySQL_Logger::processEvents() is responsible for dumping mysql query events
from the circular buffer into SQLite tables.
DUMP EVENTSLOG commands trigger a call to processEvents():
* DUMP EVENTSLOG FROM BUFFER TO MEMORY: copies events to the in-memory SQLite table
* DUMP EVENTSLOG FROM BUFFER TO DISK: copies events to the on-disk SQLite table
* DUMP EVENTSLOG FROM BUFFER TO BOTH: copies events to both in-memory and on-disk
       SQLite tables simultaneously

Added new Admin variable stats_mysql_eventslog_sync_buffer_to_disk .
This variable defines how frequently mysql query eventslog are
automatically dumped to disk.

In MySQL_Event copy constructor, queries are copied up to
eventslog_buffer_max_query_length bytes.

Added enhancements:
* added mysql-eventslog_table_memory_size
* liste columns names in insertMysqlEventsIntoDb()
* added counters and metrics (not exported yet)

Added function MySQL_Logger::getAllMetrics() to export all metrics
related to eventlogs

Adding logging for errors generated directly directly by ProxySQL

New `errno` column in tables:
- stats_mysql_query_events
- history_mysql_query_events

Logging into `stats_mysql_errors` also errors generated by ProxySQL (no backend)

Added initial TAP test to be extended