tracing: fix span initial time calculation

the previous system was not taking into account ping time, or massive reconnection time, as the init time was recorded as the time the request was being sent.

This has been fixed by simplifying the calculation for request as starting when the first bytes are buffered to be sent; then, if the request is involved in a cold connection which needs to connect, then the several handshakes (TCP/TLS/HTTP-2) are taken into account and calculated accordingly.