Add all DeFi balances querying

[LefterisJP]

WIP.

Fix backend part of #1124

[codecov]

Codecov Report

Merging #1126 into develop will increase coverage by 0.12%.
The diff coverage is 72.72%.

@@             Coverage Diff             @@
##           develop    #1126      +/-   ##
===========================================
+ Coverage    73.40%   73.53%   +0.12%     
===========================================
  Files          229      229              
  Lines        15119    15151      +32     
  Branches      2143     2149       +6     
===========================================
+ Hits         11098    11141      +43     
+ Misses        3439     3431       -8     
+ Partials       582      579       -3     

Flag	Coverage Δ
#backend	81.12% <72.72%> (+0.14%)	⬆️
#frontend_integration	54.56% <ø> (ø)
#frontend_unit	19.25% <ø> (ø)

Impacted Files	Coverage Δ
rotkehlchen/api/server.py	82.27% <ø> (ø)
rotkehlchen/api/rest.py	85.35% <57.89%> (-0.67%)	⬇️
rotkehlchen/chain/ethereum/zerion.py	92.30% <83.33%> (+0.52%)	⬆️
rotkehlchen/api/v1/resources.py	97.62% <100.00%> (+0.03%)	⬆️
rotkehlchen/serialization/serialize.py	100.00% <100.00%> (ø)
rotkehlchen/premium/premium.py	78.28% <0.00%> (-5.93%)	⬇️
rotkehlchen/chain/manager.py	84.02% <0.00%> (-0.66%)	⬇️
rotkehlchen/exchanges/poloniex.py	76.60% <0.00%> (+0.29%)	⬆️
rotkehlchen/exchanges/bittrex.py	81.77% <0.00%> (+0.46%)	⬆️
rotkehlchen/externalapis/etherscan.py	72.09% <0.00%> (+0.58%)	⬆️
... and 5 more

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