src: Avoid to call costly evbuffer_add() repeatedly

The profiler and benchmarking showed that calling evbuffer_add()
repeatedly is very costly. To avoid this, we buffer up small writes
into one large chunk and call evbuffer_add() less times.
nghttpd: More SSL_CTX options and support EDCHE cipher
nghttpd: Emit protocol id error only when verbose output is enabled
h2load: Support parellel native threads execution with -t option