Datapoint source redundancy. Make requests async and aggregate multiple sources into a single datapoint.

[greenhat]

Close #133 Close #217

[coveralls]

Pull Request Test Coverage Report for Build 3993933521

• 65 of 117 (55.56%) changed or added relevant lines in 12 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.2%) to 61.754%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/src/datapoint_source/aggregator.rs	15	16	93.75%
core/src/datapoint_source/assets_exchange_rate.rs	2	4	50.0%
core/src/datapoint_source/ada_usd.rs	2	5	40.0%
core/src/datapoint_source/erg_xau.rs	2	5	40.0%
core/src/datapoint_source/coincap.rs	11	16	68.75%
core/src/datapoint_source/predef.rs	0	5	0.0%
core/src/datapoint_source.rs	0	6	0.0%
core/src/datapoint_source/custom_ext_script.rs	0	8	0.0%
core/src/datapoint_source/coingecko.rs	28	37	75.68%
core/src/main.rs	0	10	0.0%
<!--	Total:	65	117	55.56%	-->

Files with Coverage Reduction	New Missed Lines	%
core/src/pool_commands.rs	1	0%
core/src/main.rs	2	0%
<!--	Total:	3		-->

Totals
Change from base Build 3946567315:	0.2%
Covered Lines:	2007
Relevant Lines:	3250

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💛 - Coveralls