rename the Session to Connection

[marten-seemann]

The term "session" is a remnant from gQUIC days. The QUIC RFC doesn't talk about "sessions", only about connections.

Note that this will be a quite disruptive change for downstream users.

[codecov]

Codecov Report

Merging #3361 (4b17343) into master (42f3159) will not change coverage.
The diff coverage is 84.91%.

@@           Coverage Diff           @@
##           master    #3361   +/-   ##
=======================================
  Coverage   85.61%   85.61%           
=======================================
  Files         135      135           
  Lines        9869     9869           
=======================================
  Hits         8449     8449           
  Misses       1043     1043           
  Partials      377      377           

Impacted Files	Coverage Δ
http3/roundtrip.go	93.33% <ø> (ø)
multiplexer.go	94.44% <ø> (ø)
sys_conn.go	84.62% <ø> (ø)
sys_conn_df_linux.go	52.94% <ø> (ø)
sys_conn_df_windows.go	52.94% <ø> (ø)
sys_conn_oob.go	71.88% <ø> (ø)
sys_conn_windows.go	62.50% <ø> (ø)
closed_conn.go	79.41% <33.33%> (ø)
send_conn.go	47.37% <50.00%> (ø)
http3/server.go	73.24% <81.25%> (ø)
... and 5 more

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[lucas-clemente]

Do we want any disambiguation from all the conn_* files? With this, we use "conn" for both classes of connections we have. (Maybe rename the conn_* stuff to rawConn, udpConn, sysConn?)