Rework caching to use (possibly local) .json files

[saraedum]

fixes #39.

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Dependencies

• [x] https://github.com/flatsurf/flatsurf/pull/310 i.e. https://github.com/conda-forge/flatsurf-feedstock/pull/7
• [x] We need https://github.com/conda-forge/sagelib-feedstock/pull/143 to finish the libflint and arb migrations
• [x] We need https://github.com/conda-forge/eclib-feedstock/runs/8195240504 so we finish the arb migration for sagelib.
• [x] We need https://github.com/flatsurf/flatsurf/pull/319 to be merged and released.

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TODO

• [x] Add creation & consumption of cache to CI. See #43
• [x] Increase queue size (and make it configurable/adaptive). Currently, running boshernitzan conjecture search, runs a bit low on the queue starting at about a+b+c=140. For similar a, b, c, creating a surface takes about 12 times the time than it does to check the conjecture. So we do not run at full load when running on more than 13 cores. (Alternatively, we could fill the queue in parallel, but this probably requires #42 to do properly.)
• [x] Verify that the small orbit closure surveys for 3 and 4 gons still work correctly (currently, they don't; we're doing the wrong thing for (2, 2, 3) somehow.) See https://sagemath.zulipchat.com/#narrow/stream/271193-polygon/topic/orbit.20closure.20dimension.20bound/near/298821118
• [x] Boshernitzan-conjecture crashes for (1, 1, 179) with a maximum-recursion-depth exceeded.
• [x] Ensure that orbit closure JSON report contains timestamps
• [x] Ensure that orbit closure JSON report contains only the final verdict.