Assign DataChannel ID directly to fix conflicts

[kylecarbs]

When opening DataChannels in parallel it was possible for multiple to be allocated the same ID since the generateAndSetDataChannelID func references the DataChannel ID, which was being set after the unlock.

I tested opening 1000 DataChannels at once, and it seems to work perfectly now!

[codecov[bot]]

Codecov Report

Merging #2248 (56da953) into master (589d67e) will decrease coverage by 0.14%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2248      +/-   ##
==========================================
- Coverage   77.43%   77.29%   -0.15%     
==========================================
  Files          87       87              
  Lines        9197     9195       -2     
==========================================
- Hits         7122     7107      -15     
- Misses       1642     1654      +12     
- Partials      433      434       +1     

Flag	Coverage Δ
go	79.05% <100.00%> (-0.17%)	:arrow_down:
wasm	70.16% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
datachannel.go	82.77% <100.00%> (-0.10%)	:arrow_down:
sctptransport.go	77.90% <0.00%> (-2.63%)	:arrow_down:
dtlstransport.go	63.04% <0.00%> (-1.87%)	:arrow_down:

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[Sean-Der]

Is it safe to modify d.id without holding the lock? Maybe we could make the id a atomic.Value? Then we drop the unlock and lock