make stream-attacher (and other user-callbacks) work properly with async/await

[meejah]

Note to self: look at fixing maybeDeferred in Twisted instead...

[coveralls]

Coverage decreased (-0.2%) to 99.61% when pulling cb82ab6043f91efc4d990351b3d3743a340300c8 on py3-async-def-fixes into 3cdef33e32da61a01cb5d1ee7892a61396075bcf on master.

[coveralls]

Coverage increased (+0.0008%) to 99.833% when pulling 219775fb1de37dbe64f61469dc9b98776d253643 on py3-async-def-fixes into 3cdef33e32da61a01cb5d1ee7892a61396075bcf on master.

[coveralls]

Coverage increased (+0.0008%) to 99.833% when pulling 219775fb1de37dbe64f61469dc9b98776d253643 on py3-async-def-fixes into 3cdef33e32da61a01cb5d1ee7892a61396075bcf on master.

[codecov-io]

Codecov Report

Merging #223 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #223      +/-   ##
==========================================
+ Coverage   99.83%   99.83%   +<.01%     
==========================================
  Files          20       20              
  Lines        3571     3588      +17     
==========================================
+ Hits         3565     3582      +17     
  Misses          6        6

Impacted Files	Coverage Δ
txtorcon/torstate.py	100% <100%> (ø)	:arrow_up:
txtorcon/socks.py	100% <100%> (ø)	:arrow_up:
txtorcon/torcontrolprotocol.py	100% <100%> (ø)	:arrow_up:
txtorcon/util.py	98.2% <100%> (+0.05%)	:arrow_up:

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[coveralls]

Coverage increased (+0.0008%) to 99.833% when pulling 3b9feb7f53fdf8b10f5b0d7625435b4849b820cc on py3-async-def-fixes into e76372333d70d2695fbce6ae34feefb9b551d934 on master.