[sctp] make write sync

[melekes]

There's no reason for it to be async because it just buffers the data in memory (actual IO is happening in a separate thread).

[codecov]

Codecov Report

Base: 59.36% // Head: 59.52% // Increases project coverage by +0.15% 🎉

Coverage data is based on head (bf7cd4c) compared to base (5b79f08).
Patch coverage: 76.36% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #344      +/-   ##
==========================================
+ Coverage   59.36%   59.52%   +0.15%     
==========================================
  Files         503      503              
  Lines       47666    47646      -20     
  Branches    12668    12583      -85     
==========================================
+ Hits        28299    28362      +63     
- Misses       9898     9902       +4     
+ Partials     9469     9382      -87     

Impacted Files	Coverage Δ
sctp/examples/ping.rs	0.00% <0.00%> (ø)
sctp/examples/pong.rs	0.00% <0.00%> (ø)
sctp/src/stream/stream_test.rs	47.54% <0.00%> (-0.04%)	⬇️
.../association_internal/association_internal_test.rs	60.27% <33.33%> (+0.69%)	⬆️
sctp/src/association/association_test.rs	57.98% <68.08%> (+1.93%)	⬆️
sctp/src/queue/pending_queue.rs	88.57% <100.00%> (+10.79%)	⬆️
sctp/src/queue/queue_test.rs	68.65% <100.00%> (+6.72%)	⬆️
interceptor/src/stream_reader.rs	28.57% <0.00%> (-14.29%)	⬇️
sctp/src/timer/ack_timer.rs	79.31% <0.00%> (-3.45%)	⬇️
srtp/src/stream.rs	45.23% <0.00%> (-2.39%)	⬇️
... and 7 more

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[k0nserv]

LGTM