[quicwg/base-drafts] Use a higher seed RTT for new paths (#2789)

[Martin Thomson <[e-mail]>]

@gorryfair mentions on list:

>>   A connection MAY use the delay between sending a PATH_CHALLENGE and 
receiving a PATH_RESPONSE to seed initial_rtt for a new path, but the delay 
SHOULD NOT be considered an RTT sample.
>
> .... Maybe I am wrong, but I interpret RFC6298 as saying the seed value 
should be 1.5 x larger than this RTT value?
>
> (To account for RTT variance.)

I don't quite get the same reading from 6258 (the 0.5 seems to come from setting RTTVAR on the first sample, but that isn't fed into SRTT anywhere...), but this seems like a good idea, if only out of caution.  The new path might be longer.  But it's hard to say by how much.  Some hedging seems wise.  

The first RTT measurement will erase this, so it will only govern behaviour on unresponsive paths.  So it's probably not critical.

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