[PATCH 0 of 4] error_log rate limiting

[Maxim Dounin]

Hello!

The following patch series introduces error_log rate limiting,
with the default rate limit set to 1000 messages per second (for each
worker process).

Rate limiting uses "leaky bucket" algorithm (as in the limit_req module)
with burst set to a value proportional to the log message severity,
so more severe messages can be logged even if less severe are already
rate-limited.  That is, rate limiting more or less dynamically adjusts
log level to keep logging rate under the specified limit.

Review and testing appreciated.

-- 
Maxim Dounin