move GSO control message handling to the oobConn

[marten-seemann]

For #3999. For ECN, we'll need to append even more control messages, and the current construction would get too complicated.

[codecov]

Codecov Report

Merging #4056 (17e9480) into master (2797f85) will increase coverage by 0.62%.
Report is 1 commits behind head on master.
The diff coverage is 82.35%.

@@            Coverage Diff             @@
##           master    #4056      +/-   ##
==========================================
+ Coverage   83.29%   83.91%   +0.62%     
==========================================
  Files         147      147              
  Lines       14908    15104     +196     
==========================================
+ Hits        12417    12674     +257     
+ Misses       1990     1933      -57     
+ Partials      501      497       -4     

Files Changed	Coverage Δ
packet_handler_map.go	84.52% <ø> (ø)
sys_conn.go	77.19% <33.33%> (-2.81%)	⬇️
sys_conn_oob.go	78.19% <66.67%> (+7.70%)	⬆️
transport.go	81.72% <66.67%> (+13.22%)	⬆️
send_conn.go	87.23% <90.00%> (+1.78%)	⬆️
connection.go	72.57% <100.00%> (ø)
send_queue.go	100.00% <100.00%> (ø)
server.go	83.24% <100.00%> (+7.35%)	⬆️

... and 2 files with indirect coverage changes

[sukunrt]

LGTM. Just this one comment: #4056 (comment)