Move some Policy settings to config

[fyrchik]

Close #1742 .

1. MaxTransactionsPerBlock is moved to config.
2. MaxBlockSize and MaxBlockSystemFee are removed. See also https://github.com/neo-project/neo/pull/2300/files#r571941755

[codecov]

Codecov Report

Merging #1759 (ffd85dd) into master (924de3e) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1759      +/-   ##
==========================================
+ Coverage   83.54%   83.61%   +0.06%     
==========================================
  Files         273      273              
  Lines       22103    21980     -123     
==========================================
- Hits        18467    18379      -88     
+ Misses       2509     2478      -31     
+ Partials     1127     1123       -4     

Impacted Files	Coverage Δ
pkg/consensus/consensus.go	68.55% <ø> (-2.02%)	⬇️
pkg/interop/native/policy/policy.go	0.00% <ø> (ø)
pkg/rpc/client/policy.go	62.16% <ø> (-1.95%)	⬇️
pkg/core/blockchain.go	77.82% <100.00%> (+0.08%)	⬆️
pkg/core/native/policy.go	86.78% <100.00%> (+0.79%)	⬆️
pkg/services/notary/notary.go	91.37% <0.00%> (-1.02%)	⬇️
pkg/services/oracle/request.go	62.29% <0.00%> (+4.91%)	⬆️
pkg/services/oracle/oracle.go	90.58% <0.00%> (+11.76%)	⬆️

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