main request to location "/drizzle" (handled by mod_drizzle)
	-> mod_drizzle content handler (is main request)
		-> emit subrequest to location "/drizzle"
		-> return NGX_DONE
	-> mod_drizzle content handler (is subrequest)
		-> connect db (through libdrizzle), get db connection fd
		-> wrap db connection fd into a ngx_event, reg rd/wr event handler
		-> return NGX_DONE
	-> i/o event occured on db connection
		-> rd/wr event handler called, call mod_drizzle content handler using subrequest struct
			-> mod_drizzle content handler called (is subrequest)
				-> continue libdrizzle state machine
				-> generate response header and body if done,
					then unreg db connection events,
					and call ngx_http_finalize_request to release self
				-> return NGX_OK