normalize use of backtrack, last_event_id and if_modified_since/if_none_match values to get old messages on all subscriber modes

removed push_stream_content_type directive, use default_type Nginx directive to set the value, except on JSONP and Event Source modes

removed push_stream_keepalive directive, let Nginx decide when to use keepalive and how many requests accept using keepalive_* directives

remove push_stream_shared_memory_cleanup_objects_ttl directive, using default value 10s, and 4s for cleanup timer

change push_stream_eventsource_support directive to be a subtype of push_stream_subscriber directive