Collapse pending/active/capacity maps in connstate.State

[codygibb]

Instead of managing three maps for pending/active/capacity (which was complex in it's own
respect), just have a single map with a status field, and then group conns by torrent. Allows us
to ask questions like "how many active conns does this torrent have?" without having to loop
through a global active conns map.

This is shown by the new Saturated function.

[yiranwang52]

can you add some comments about what each means

[codygibb]

I think pending / active are fairly obvious in the context of this file... There's a big comment above State which describes the semantics of pending / active.

[codecov-io]

Codecov Report

Merging #173 into master will decrease coverage by 0.1%.
The diff coverage is 89.33%.

@@            Coverage Diff             @@
##           master     #173      +/-   ##
==========================================
- Coverage   67.38%   67.27%   -0.11%     
==========================================
  Files         180      180              
  Lines        8805     8811       +6     
==========================================
- Hits         5933     5928       -5     
- Misses       2122     2133      +11     
  Partials      750      750

Impacted Files	Coverage Δ
lib/torrent/scheduler/dispatch/dispatcher.go	49.33% <ø> (-3.74%)	⬇️
lib/torrent/scheduler/events.go	71.15% <100%> (-1.58%)	⬇️
lib/torrent/scheduler/connstate/state.go	90.76% <88.73%> (+0.25%)	⬆️
core/peer_info.go	88.88% <0%> (+5.55%)	⬆️

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