Implement DNS seed

[mkykadir]

Fixes #2874

Adds DNS seed support like Bitcoin seeding. Parameters can be tweaked.

• Adds new config field to Registry DNS seed update interval
• A or AAAA query will return randomly selected validator nodes
• Maximum of 10 nodes will be returned to seed queries

[codecov[bot]]

Codecov Report

Merging #3320 (0212e95) into v0.x.x (09895d4) will increase coverage by 0.76%. The diff coverage is 52.00%.

@@            Coverage Diff             @@
##           v0.x.x    #3320      +/-   ##
==========================================
+ Coverage   87.74%   88.51%   +0.76%     
==========================================
  Files         165      165              
  Lines       17107    17131      +24     
==========================================
+ Hits        15011    15163     +152     
+ Misses       2096     1968     -128     

Flag	Coverage Δ
integration	34.29% <32.00%> (-0.15%)	:arrow_down:
unittests	87.52% <52.00%> (-0.22%)	:arrow_down:

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Impacted Files	Coverage Δ
source/agora/node/Config.d	68.61% <ø> (+3.64%)	:arrow_up:
source/agora/node/Registry.d	75.94% <52.00%> (-1.35%)	:arrow_down:
source/agora/consensus/BlockStorage.d	68.90% <0.00%> (-10.61%)	:arrow_down:
source/agora/test/ValidatorRecurringEnrollment.d	94.23% <0.00%> (-0.97%)	:arrow_down:
source/agora/consensus/protocol/Nominator.d	91.11% <0.00%> (-0.89%)	:arrow_down:
source/agora/consensus/Ledger.d	97.41% <0.00%> (-0.15%)	:arrow_down:
source/agora/consensus/state/Ledger.d	89.50% <0.00%> (+0.65%)	:arrow_up:
source/agora/node/Validator.d	92.46% <0.00%> (+1.00%)	:arrow_up:
source/agora/network/Manager.d	77.60% <0.00%> (+1.26%)	:arrow_up:
source/agora/node/FullNode.d	74.40% <0.00%> (+3.57%)	:arrow_up:
... and 5 more

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