Add key exchange primitives for p2p

[anjmao]

Add primitives to support shared key exchange using NACL. The idea is that both provider and consumer need to exchange public key pairs which are signed by etherium private keys. Once exchange is done p2p will use Salsa20 cipher for data encryption.

Exchange itself will be implemented in separate PR using NATS broker.

[codecov-io]

Codecov Report

Merging #1884 into master will increase coverage by 0.06%.
The diff coverage is 74.13%.

@@            Coverage Diff             @@
##           master    #1884      +/-   ##
==========================================
+ Coverage   47.05%   47.12%   +0.06%     
==========================================
  Files         288      288              
  Lines       11775    11806      +31     
==========================================
+ Hits         5541     5563      +22     
- Misses       5799     5804       +5     
- Partials      435      439       +4     

Impacted Files	Coverage Δ
p2p/key.go	62.50% <62.50%> (-37.50%)	⬇️
p2p/channel.go	74.10% <88.46%> (-2.53%)	⬇️
tequilapi/sse/sse.go	74.44% <0.00%> (+3.33%)	⬆️
core/policy/oracle.go	91.89% <0.00%> (+5.40%)	⬆️

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