graceful shutdown of websocket

[valli0x]

checking for ctx.Done in a websockets subscription. It needed for graceful shutdown.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 13 lines in your changes missing coverage. Please review.

Project coverage is 46.16%. Comparing base (e567c5a) to head (852b2d9).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
rpc/websockets.go	0.00%	12 Missing ⚠️
server/json_rpc.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #547      +/-   ##
===========================================
- Coverage    46.17%   46.16%   -0.01%     
===========================================
  Files          181      181              
  Lines        18122    18125       +3     
===========================================
  Hits          8368     8368              
- Misses        9037     9040       +3     
  Partials       717      717              

Files with missing lines	Coverage Δ
server/json_rpc.go	0.00% <0.00%> (ø)
rpc/websockets.go	0.00% <0.00%> (ø)