The PCRE JIT compiler uses mmap to allocate memory for its executable codes, so
we have to explicitly call the pcre_free_study() function to free this memory.

Example configuration to reproduce:

  server {
      proxy_redirect off;

      location / {
          proxy_pass http://localhost:8000;
          proxy_redirect http://localhost:8000/ /;

          location ~ \.php$ {
              proxy_pass http://localhost:8000;
              # proxy_redirect must be inherited from the level above,
              # but instead it was switched off here
          }
      }
  }

Previously if ngx_add_event() failed a connection was freed two times (once
in the ngx_event_connect_peer(), and again by a caller) as pc->connection was
left set.  Fix is to always use ngx_close_connection() to close connection
properly and set pc->connection to NULL on errors.

Patch by Piotr Sikora.

Doing a cleanup before every lookup seems to be too aggressive. It can lead to
premature removal of the nodes still usable, which increases the amount of work
under a mutex lock and therefore decreases performance.

In order to improve cleanup behavior, cleanup function call has been moved right
before the allocation of a new node.

function.

No functional changes.

"limit_req_zone" directive; minimum size of zone is increased.

Previously an unsigned variable was used to keep the return value of
ngx_parse_size() function, which led to an incorrect zone size if NGX_ERROR
was returned.

The new code has been taken from the "limit_conn_zone" directive.