MINOR: mux-quic/h3: emit SETTINGS via MUX tasklet handler

MINOR: mux-quic/h3: emit SETTINGS via MUX tasklet handler

Previously, QUIC MUX application layer was installed and initialized via
MUX init. However, the latter stage involve I/O operations, for example
when using HTTP/3 with the emission of a SETTINGS frame.

Change this to prevent any I/O operations during MUX init. As such,
finalize app_ops callback is now called during the first invokation of
qcc_io_send(), in the context of MUX tasklet. To implement this, a new
application state value is added, to detect the transition from NULL to
INIT stage.