Update linters and fix reported issues

[Danielius1922]

Removed linters:

• deadcode (obsolete)
• varcheck (obsolete)

Enabled linters:

• forcetypeassert
• gocritic
• gocyclo
• gomoddirectives
• gomodguard
• nilerr
• revive
• unparam

[codecov-commenter]

Codecov Report

Merging #393 (8725987) into v3 (ba2022c) will increase coverage by 0.08%. The diff coverage is 78.87%.

@@            Coverage Diff             @@
##               v3     #393      +/-   ##
==========================================
+ Coverage   70.26%   70.35%   +0.08%     
==========================================
  Files          67       67              
  Lines        5085     5110      +25     
==========================================
+ Hits         3573     3595      +22     
+ Misses       1131     1128       -3     
- Partials      381      387       +6     

Impacted Files	Coverage Δ
dtls/server/config.go	64.28% <ø> (ø)
message/getmid.go	80.00% <0.00%> (ø)
options/udpOptions.go	34.61% <ø> (ø)
udp/client/config.go	75.00% <ø> (+8.33%)	:arrow_up:
udp/server/config.go	71.42% <ø> (ø)
message/pool/pool.go	84.00% <33.33%> (-7.31%)	:arrow_down:
dtls/server/server.go	77.03% <57.14%> (+0.95%)	:arrow_up:
net/connUDP.go	61.32% <71.42%> (-0.68%)	:arrow_down:
tcp/coder/coder.go	70.92% <71.42%> (-0.51%)	:arrow_down:
tcp/server/server.go	77.51% <71.42%> (+1.00%)	:arrow_up:
... and 17 more

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