Implements feature 'Client Error Limit'

[JavierJF]

Feature description

'Client Error Limit' is a feature that limits the number of connection errors that can be initiated from a particular address before denying future connections from that address.

Feature is controlled by new introduced variables:

• mysql-client_host_error_counts: Determines the number of errors than a client address needs to reach before getting banned for future connections.
• mysql-client_host_cache_size: Determines the size of the cache being used for keeping track of the clients addresses that have had any connection errors.
• Command 'PROXYSQL FLUSH MYSQL CLIENT HOSTS': Clears the client host cache.

When enabled, the expected behavior for the cache is the following:

When a new connection arrives, the cache is check for knowing if the client address has reached the
maximum errors, if the limit has been reached, and error is printed in ProxySQL log advertising the
event. If the limit is not reached, the connection tries to be performed, the situation has now to
possible outcomes:

a) The connection fails, the client address is added to the cache if it wasn't present, otherwise,
   it's error count is increased. If the cache was full, the oldest entry will be replaced with one
   holding this client address.
b) The connection succeeds, if the client address was present in the cache, it's removed from it.

Two new tables has also been introduced for being able to inspect the client_host_cache :

• stats.stats_mysql_client_host_cache
• stats.stats_mysql_client_host_cache_reset

As in other tables, querying stats_mysql_client_host_cache_reset performs a reset of the current values hold by the cache.

[JavierJF]

Retest this please.