use new sync.Map instead of handler container

[jkralik]

No description provided.

[codecov-commenter]

Codecov Report

Merging #358 (1df4040) into v3 (3a905a5) will decrease coverage by 0.27%.
The diff coverage is 68.85%.

@@            Coverage Diff             @@
##               v3     #358      +/-   ##
==========================================
- Coverage   70.60%   70.32%   -0.28%     
==========================================
  Files          67       65       -2     
  Lines        5556     5480      -76     
==========================================
- Hits         3923     3854      -69     
- Misses       1237     1242       +5     
+ Partials      396      384      -12     

Impacted Files	Coverage Δ
udp/discover.go	53.48% <50.00%> (-1.06%)	⬇️
udp/client/clientconn.go	72.34% <57.89%> (-0.03%)	⬇️
tcp/clientconn.go	67.82% <60.00%> (+1.26%)	⬆️
tcp/clientobserve.go	64.76% <71.42%> (-1.59%)	⬇️
udp/client/clientobserve.go	68.36% <71.42%> (-0.57%)	⬇️
tcp/session.go	70.77% <80.00%> (-0.08%)	⬇️
dtls/client.go	76.31% <100.00%> (ø)
dtls/server.go	84.00% <100.00%> (-1.72%)	⬇️
tcp/server.go	81.28% <100.00%> (ø)
udp/client.go	86.66% <100.00%> (ø)
... and 6 more