Fix race conditions in AMQPServer, TopicExchange & Binding

[cjorba]

• Added mutex for TopicExchange for synchronization.
• Added mutex for Binding for synchronization.
• Added mutex for AMQPServer for synchronization.

[codecov-io]

Codecov Report

Merging #28 into master will increase coverage by 0.22%.
The diff coverage is 68.18%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
+ Coverage   48.45%   48.67%   +0.22%     
==========================================
  Files          20       20              
  Lines        1197     1210      +13     
==========================================
+ Hits          580      589       +9     
- Misses        593      597       +4     
  Partials       24       24

Impacted Files	Coverage Δ
amqptest/server/server.go	3.6% <0%> (-0.04%)	⬇️
amqptest/server/vhost.go	59.82% <100%> (-0.68%)	⬇️
amqptest/server/exchange.go	72.91% <71.42%> (-9.44%)	⬇️
amqptest/dial.go	89.81% <0%> (+3.7%)	⬆️

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[i4ki]

Thanks @cjorba !