Extensions from nginx-retry development

[avahahn]

Proposed changes

• minimal bindings for events (nginx_event_t and company) (see example use here)
• getters for request status and subrequest depth
• cycle count increment
• flags as argument to subrequest
• subrequest can accept a custom data (see example use here)
• do not automatically nuke the subrequest body. that should be up to the caller.

Testing

• All changes tested with nginx-retry.
• Other modules have not yet been tested for compatibility with these changes.