fix: fix udp socket create error

[hsqStephenZhang]

background

in these #352 #102 issues, someone mentioned that the performance is pretty low at the master branch, i encountered the same situation at my desktop and vps, so i checked out the previous commits and located the root cause.

root cause

from commit 5a49e83, the hand-written udp socket was changed to socket2. but in peer.rs, the udp socket's creation method's argument was wrong.

fix

Changed the socket type from Type::STREAM to Type::DGRAM to correctly handle UDP connections.

tests

i have tested several commits's performance with iperf3. after the fix, the throughput was finally ok