Fully drain RFC2217 on graceful close

[puddly]

This fixes the flaky RFC2217 test that we've been seeing by aligning close semantics for RFC2217 with that of other backends. There is no form of backpressure or flow control but we get around it by using TCP and enqueuing a no-op command with a simple response upon close: when we receive a response, we know all previous data has been sent out and are good to close. abort should work as before.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.27%. Comparing base (f9f1bf2) to head (1936940).
⚠️ Report is 2 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #100      +/-   ##
==========================================
+ Coverage   92.23%   92.27%   +0.03%     
==========================================
  Files          22       22              
  Lines        3632     3649      +17     
==========================================
+ Hits         3350     3367      +17     
  Misses        282      282              

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