The RateLimit class stores authentication failure data mapped by the
client IP addresses. The authentication failures are limited
per thread. The limits are still hard-coded and at least the number of
failures should be made configurable.
The simplest, most maintainable and acceptably efficient implementation
for DDoS protection is a thread-local unordered_map. The unwanted
side-effect of "scaling" of the number of allowed authentication failures
is unlikely to be problematic in most use-cases.
As the blocking of a host is only temporary, the behavior differs from the
one in the MariaDB server. This allows the number of failures to be set to
a much lower value negating some of the problems caused by the relatively
simple implementation.
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