network: make GossipNode more independent from wsNetwork implementation

[cce]

Summary

This removes some unused interface methods on GossipNode, and extracts some functionality from WebsocketNetwork into two new types (msgHandler, msgBroadcaster) to make them more available for re-use.

Test Plan

Existing tests should pass.

[codecov]

Codecov Report

Merging #5634 (0d03251) into master (8b19c62) will increase coverage by 0.02%.
Report is 1 commits behind head on master.
The diff coverage is 79.04%.

@@            Coverage Diff             @@
##           master    #5634      +/-   ##
==========================================
+ Coverage   54.95%   54.97%   +0.02%     
==========================================
  Files         463      463              
  Lines       64476    64494      +18     
==========================================
+ Hits        35432    35458      +26     
+ Misses      26661    26656       -5     
+ Partials     2383     2380       -3     

Files Changed	Coverage Δ
network/wsPeer.go	73.42% <53.84%> (+2.58%)	⬆️
network/wsNetwork.go	73.03% <92.59%> (+0.16%)	⬆️
network/msgCompressor.go	85.71% <100.00%> (ø)
network/multiplexer.go	95.34% <100.00%> (-0.11%)	⬇️
network/netidentity.go	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[algorandskiy]

changes makes sense for me although some implementation notes are below.

[iansuvak]

LGTM in general, I am pro moving interfaces out of implementation specific wsNetwork.go file but doesn't need to happen here.