MQTT performance optimization

[suchen-sci]

This pr contains following optimizations and updates for MQTT proxy.

1. remove unnecessary defer
2. remove client writePacket lock since golang write to Conn already contain locks.
3. since client writePacket has no lock, then cancel create goroutine in session.publish (no risk of dead lock), use buffered channel instead.
4. use github.com/hashicorp/golang-lru package in topic cache. Keep both speed and memory security.
5. merge topic split and validation check into one function to speed up. (new splitTopic function in topic.go)
6. drop qos0 message when sending channel is full.
7. clean session from DB when cleanSession flag is set
8. clean topicMgr when client is closed to improve performance.

Thanks a lot for advice from @faceair.

[megaeasex]

[TASK:easegress-pr-test SUCCESS]megaease/easegress Pull Request 304 Deploy Test Success

[megaeasex]

[TASK:easegress-pr-test SUCCESS]megaease/easegress Pull Request 304 Deploy Test Success

[codecov-commenter]

Codecov Report

Merging #304 (fd29438) into main (c0f5200) will increase coverage by 0.25%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##             main     #304      +/-   ##
==========================================
+ Coverage   80.22%   80.47%   +0.25%     
==========================================
  Files          53       53              
  Lines        5897     5973      +76     
==========================================
+ Hits         4731     4807      +76     
- Misses        907      908       +1     
+ Partials      259      258       -1     

Impacted Files	Coverage Δ
pkg/object/mqttproxy/spec.go	100.00% <ø> (ø)
pkg/object/mqttproxy/session_manager.go	86.36% <40.00%> (-3.81%)	⬇️
pkg/object/mqttproxy/session.go	82.31% <66.66%> (+0.83%)	⬆️
pkg/object/mqttproxy/storage.go	94.87% <77.77%> (-5.13%)	⬇️
pkg/object/mqttproxy/client.go	81.09% <80.76%> (+2.15%)	⬆️
pkg/object/mqttproxy/broker.go	79.68% <90.00%> (+2.35%)	⬆️
pkg/object/mqttproxy/topic.go	100.00% <100.00%> (+0.46%)	⬆️
pkg/object/meshcontroller/spec/spec.go	89.52% <0.00%> (+0.01%)	⬆️
... and 2 more

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[megaeasex]

[TASK:easegress-pr-test SUCCESS]megaease/easegress Pull Request 304 Deploy Test Success

[megaeasex]

[TASK:easegress-pr-test SUCCESS]megaease/easegress Pull Request 304 Deploy Test Success

[megaeasex]

[TASK:easegress-pr-test SUCCESS]megaease/easegress Pull Request 304 Deploy Test Success

[megaeasex]

[TASK:easegress-pr-test SUCCESS]megaease/easegress Pull Request 304 Deploy Test Success