Add start timeout to streaming profile

Add start timeout to streaming profile

This allows overriding the hardcoded grace period of 20 seconds.
It should address the problems described in [1][2].

In addition, timeout code has been slightly refactored for readability
and more debug logging.

[1] https://tvheadend.org/d/8330-increase-timeout-when-tuning-iptv-mux/2
[2] https://tvheadend.org/d/8158-several-problems-questions-about-using-tvheadend-starting-with-not-waiting-long-enoough-for-stream-to-begin