Added WsCombinedTradeServe(...)

[tgbv]

Addresses issue #409

• Added binance.WsCombinedTradeServe(...) as combined alternative for binance.WsTradeServe(...)
• Added test for binance.WsCombinedTradeServe(...)

[codecov[bot]]

Codecov Report

Merging #410 (a7dd531) into master (0e882f1) will increase coverage by 0.01%. The diff coverage is 78.57%.

@@            Coverage Diff             @@
##           master     #410      +/-   ##
==========================================
+ Coverage   69.98%   69.99%   +0.01%     
==========================================
  Files          74       74              
  Lines        6819     6833      +14     
==========================================
+ Hits         4772     4783      +11     
- Misses       1548     1550       +2     
- Partials      499      500       +1     

Impacted Files	Coverage Δ
v2/websocket_service.go	77.71% <78.57%> (+0.03%)	:arrow_up:

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