feat: fetch minimal data and cache where possible

[satello]

• finding all open disputes in a session requires looping over the entire disputes array. Not getting voteCounters reduced the time from ~700ms/dispute to ~200ms/dispute.
• Cache data used in fetching disputes (timestamps, open disputes, meta evidence)
• Getting disputes table should be much faster but still relatively slow (~ 200ms * number of disputes + (2-9 seconds) * number of sessions juror was drawn + some extra time for http requests, draws, dispute created event).
• Loading a dispute takes about 15 seconds.

[coveralls]

Pull Request Test Coverage Report for Build 777

• 6 of 40 (15.0%) changed or added relevant lines in 4 files are covered.
• 9 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.09%) to 16.327%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/resources/Disputes.js	0	1	0.0%
src/contracts/implementations/arbitrable/Arbitrable.js	1	4	25.0%
src/utils/StoreProviderWrapper.js	1	4	25.0%
src/contracts/implementations/arbitrator/Kleros.js	4	31	12.9%

Files with Coverage Reduction	New Missed Lines	%
src/utils/StoreProviderWrapper.js	1	2.86%
src/resources/Disputes.js	1	5.26%
src/contracts/implementations/arbitrable/Arbitrable.js	2	5.26%
src/contracts/implementations/arbitrator/Kleros.js	5	2.45%

Totals
Change from base Build 770:	-0.09%
Covered Lines:	201
Relevant Lines:	1001

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💛 - Coveralls