[quicwg/base-drafts] Simplify the client's PTO code by allowing the server to send a PING (#3161)

[ianswett <[e-mail]>]

At the New York QUIC interim, we dealt with the problem of the server being blocked by the amplification factor by making it arm the PTO until it knew the server had validated the path, even if it had nothing to send.

>From the recovery [pseudocde](https://github.com/quicwg/base-drafts/blob/master/draft-ietf-quic-recovery.md#on-timeout):
`  if (endpoint is client without 1-RTT keys):
    // Client sends an anti-deadlock packet: Initial is padded
    // to earn more anti-amplification credit,
    // a Handshake packet proves address ownership.
    if (has Handshake keys):
      SendOneAckElicitingHandshakePacket()
    else:
      SendOneAckElicitingPaddedInitialPacket()`

When reviewing https://github.com/quicwg/base-drafts/pull/3057, I realized that if we allowed the server to send a PING-only packet when the PTO fired and the amplification factor had been reached, then that would elicit a padded Initial ACK frame in response, which also fixes the issue.

@martinduke though that was a simpler solution than the current approach and this is a bit of an edge case, so simple is probably good.


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