feat(db): testnet-golf and prevent mixing networks

[jansegre]

Acceptance criteria: after this version is in use (i.e., after nodes have upgraded), a database used on the mainnet should be rejected immediately when trying to use it for the testnet, and vice-versa; additionally when upgrading mainnet should be implied, but testnet-golf must be explicit (i.e., do not try to "import" a testnet-foxtrot database).

[codecov]

Codecov Report

Merging #342 (dcebfee) into dev (2897e77) will decrease coverage by 0.04%.
The diff coverage is 69.23%.

@@            Coverage Diff             @@
##              dev     #342      +/-   ##
==========================================
- Coverage   82.11%   82.06%   -0.05%     
==========================================
  Files         153      153              
  Lines       14464    14489      +25     
  Branches     2080     2086       +6     
==========================================
+ Hits        11877    11891      +14     
- Misses       2161     2168       +7     
- Partials      426      430       +4     

Impacted Files	Coverage Δ
hathor/conf/testnet.py	100.00% <ø> (ø)
hathor/transaction/storage/transaction_storage.py	87.05% <68.00%> (-0.59%)	⬇️
hathor/transaction/storage/exceptions.py	100.00% <100.00%> (ø)
hathor/p2p/manager.py	68.28% <0.00%> (-1.50%)	⬇️
hathor/p2p/node_sync.py	83.45% <0.00%> (ø)

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[luislhl]

That looks great