NGTCP2_ERR_RETRY

[jagerman]

In looking at ngtcp2 example/server, I came across handling of NGTCP2_ERR_RETRY being a possible return value from ngtcp2_conn_read_pkt, along with a bunch of stuff in that same example c++ code to actually build and send a retry packet.

I'm thinking we should do the same.

[jagerman]

Actually, on looking at this some more, this looks like quite a narrow edge case. Low priority.

[dr7ana]

@jagerman i knocked out a code pathway for retry tokens on initial packets, we can reuse some of the methods for the return value case from ngtcp2_conn_read_pkt