perf: fit ~20% more data into each utp packet

[carver]

See a discussion of how high the max_packet_size could go:
#767 (comment)

Here is some discussion internal to discv5, with an example of calculating a max packet size:
https://github.com/sigp/discv5/blob/d2e30e04ee62418b9e57278cee907c02b99d5bd1/src/service.rs#L1054-L1067

Another fairly reasonable approach to this would be to set the packet size too high on a local build, and look out for this warning log:
https://github.com/sigp/discv5/blob/d2e30e04ee62418b9e57278cee907c02b99d5bd1/src/socket/send.rs#L67-L68

Then we can reduce the max size to not trigger that warning.

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All of this work will probably have a limited lifetime, since we plan to switch to use this at some point: ethereum/devp2p#229

[github-actions]

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