Implement tcp_slow_start_after_idle

[Menduist]

As far as I can tell, the shadow TCP stack doesn't handle this kernel option that is enabled by default in linux.
It has a big impact on bursty traffics, since it will effectively ruin the bandwidth after an idle period > RTO.

Kernel source code:
https://github.com/torvalds/linux/blob/0b5547c51827e053cc754db47d3ec3e6c2c451d2/include/net/tcp.h#L1415-L1427
https://github.com/torvalds/linux/blob/0b5547c51827e053cc754db47d3ec3e6c2c451d2/net/ipv4/tcp.c#L674

Relevant RFCs:
https://datatracker.ietf.org/doc/html/rfc2581#section-4.1

Therefore, a TCP SHOULD set cwnd to no more than RW before beginning
transmission if the TCP has not sent data in an interval exceeding
the retransmission timeout.

[stevenengler]

Thanks for opening an issue and including the links! We're in the process of rewriting shadow's TCP code, so I'll keep this in mind and try to include it in the rewrite if there's time. It's good that we have a tracking issue for this now, so even if it doesn't get included as part of the rewrite, it should hopefully be straightforward to add later.