server: Move to using Backend.Features() instead of Enable* flags

[foxcpp]

trafficstars

Closes #107.

[codecov[bot]]

Codecov Report

Merging #112 into master will increase coverage by 0.38%. The diff coverage is 58.33%.

@@            Coverage Diff             @@
##           master     #112      +/-   ##
==========================================
+ Coverage   62.09%   62.47%   +0.38%     
==========================================
  Files           8        8              
  Lines        1108     1114       +6     
==========================================
+ Hits          688      696       +8     
+ Misses        312      311       -1     
+ Partials      108      107       -1     

Impacted Files	Coverage Δ
conn.go	59.82% <57.14%> (+0.76%)	:arrow_up:
server.go	57.27% <60.00%> (+0.12%)	:arrow_up:

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[corny]

I would like to see a func (f Feature) Contains(feat int) bool { ... } method to clean up the code.

[emersion]

I wonder how we should approach backwards compatibility. If we add a new feature, we won't be able to enable it by default without breaking the backend?

[foxcpp]

Addressed feedback: added Feature.Contains() and inverted flags (making extensions opt-in rather than opt-out).