Use POST instead of GET for historical asset price endpoint

[LefterisJP]

Reason being the formulation of the query string is hard (we don't
know how to and would need to spend time to look it up) with nested arrays.

[kelsos]

thanks, lgtm

[codecov]

Codecov Report

Merging #2250 (4187f5f) into develop (d90cd47) will increase coverage by 0.66%.
The diff coverage is 61.11%.

@@             Coverage Diff             @@
##           develop    #2250      +/-   ##
===========================================
+ Coverage    72.72%   73.39%   +0.66%     
===========================================
  Files          380      399      +19     
  Lines        22089    26389    +4300     
  Branches      3256     3989     +733     
===========================================
+ Hits         16065    19368    +3303     
- Misses        5152     5925     +773     
- Partials       872     1096     +224     

Flag	Coverage Δ
backend	80.51% <64.70%> (-1.09%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...p/src/components/dashboard/DashboardAssetTable.vue	80.00% <ø> (ø)
...d/app/src/components/dashboard/OverallBalances.vue	71.87% <ø> (-6.25%)	⬇️
...ntend/app/src/components/display/AmountDisplay.vue	95.45% <ø> (ø)
...rontend/app/src/components/helper/AssetDetails.vue	100.00% <ø> (+25.00%)	⬆️
...p/src/components/settings/AccountAssetBalances.vue	60.00% <ø> (ø)
...tend/app/src/components/settings/AssetBalances.vue	77.77% <0.00%> (ø)
rotkehlchen/csv_exporter.py	74.16% <ø> (ø)
rotkehlchen/icons.py	51.21% <25.00%> (-2.73%)	⬇️
rotkehlchen/api/v1/resources.py	94.15% <100.00%> (ø)
rotkehlchen/constants/timing.py	100.00% <100.00%> (ø)
... and 49 more

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[LefterisJP]

The backend test was an infura timeout:

2021-02-01T21:28:08.8041729Z WARNING  rotkehlchen.chain.ethereum.manager:manager.py:392 Failed to query own node for <bound method EthereumManager._call_contract of <rotkehlchen.chain.ethereum.manager.EthereumManager object at 0x7f94b2ebe350>> due to Error doing call on contract 0x06FE76B2f432fdfEcAEf1a7d4f6C3d41B5861672: {'code': -32000, 'message': 'execution aborted (timeout = 5s)'}

The e2e is the same flaky as always.