Core: added cyclic memory buffer support for error_log.

Example of usage:

  error_log memory:16m debug;

This allows to configure debug logging with minimum impact on performance.
It's especially useful when rare crashes are experienced under high load.

The log can be extracted from a coredump using the following gdb script:

  set $log = ngx_cycle->log

  while $log->writer != ngx_log_memory_writer
      set $log = $log->next
  end

  set $buf = (ngx_log_memory_buf_t *) $log->wdata
  dump binary memory debug_log.txt $buf->start $buf->end