*: add go 1.16 support

[AnnaShaleva]

Close #1787.

Depends on nspcc-dev/dbft#52.

Benchmark results summary (average TPS):

	Go 1.14	Go 1.15	Go 1.16
GoSingle 30 wrk	10597.374	10510.053	10697.590
GoFourNodes 30 work	1688.131	1954.547	1842.542

Benchmark results logs:

1_14_Go4x1_wrk_30.log
1_14_GoSingle_wrk_30.log
1_15_Go4x1_wrk_30.log
1_15_GoSingle_wrk_30.log
1_16_Go4x1_wrk_30.log
1_16_GoSingle_wrk_30.log

[codecov]

Codecov Report

Merging #1795 (6626a7d) into master (10cc19b) will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1795      +/-   ##
==========================================
+ Coverage   83.54%   83.62%   +0.08%     
==========================================
  Files         271      271              
  Lines       22026    22026              
==========================================
+ Hits        18401    18419      +18     
+ Misses       2500     2479      -21     
- Partials     1125     1128       +3     

Impacted Files	Coverage Δ
pkg/rpc/client/wsclient.go	82.70% <0.00%> (+1.50%)	⬆️
pkg/services/oracle/request.go	62.29% <0.00%> (+4.91%)	⬆️
pkg/services/oracle/oracle.go	89.87% <0.00%> (+12.65%)	⬆️

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[roman-khimov]

dbft probably needs to be updated after the merge. Fine otherwise.