sampling-based, accumulative event logging

[kazuho]

To accomplish that, we need to do the following:

• move event mask to quicly_conn_t
• let the application set the event mask when calling quicly_accept or quicly_connect
• create keyed stash for quicly_conn_t (like pthread_specific or h2o_context_get_handler_context) so that the event logger can accumulate events per connection
• create an accumulative event logger (with an option to set the maximum buffer size per connection)