simplify the connection, rename it to sendConn

[marten-seemann]

No description provided.

[codecov]

Codecov Report

Merging #2707 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2707      +/-   ##
==========================================
- Coverage   86.77%   86.75%   -0.01%     
==========================================
  Files         124      124              
  Lines        9893     9882      -11     
==========================================
- Hits         8584     8573      -11     
  Misses        979      979              
  Partials      330      330              

Impacted Files	Coverage Δ
closed_session.go	83.72% <ø> (ø)
session.go	77.35% <ø> (ø)
client.go	81.67% <100.00%> (ø)
send_conn.go	100.00% <100.00%> (ø)
send_queue.go	100.00% <100.00%> (ø)
server.go	84.15% <100.00%> (ø)

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