MINOR: mux: Make sure every string is woken up after the handshake.

MINOR: mux: Make sure every string is woken up after the handshake.

In case any stream was waiting for the handshake after receiving early data,
we have to wake all of them. Do so by making the mux responsible for
removing the CO_FL_EARLY_DATA flag after all of them are woken up, instead
of doing it in si_cs_wake_cb(), which would then only work for the first one.
This makes wait_for_handshake work with HTTP/2.