Fix cache for multiple time comparisons

[betodealmeida]

Currently we have these two conflicting behaviors:

1. When computing the cache key from the query object "we remove datetime bounds that are hard values, and replace them with the use-provided inputs to bounds, which may be time-relative (as in "5 days ago" or "now")." This removes from_dttm and to_dttm from the query object when generating the cache dict.
2. When running a query for a time comparison, say "1 week", we move the values stored in the keys from_dttm and to_dttm in the query object to inner_from_dttm and inner_to_dttm, and update the original keys with the shifted values.

When the second query runs, the cache key will be different from the first one even though from_dttm and to_dttm are stripped from both query objects, because the second one has the inner_from_dttm and inner_to_dttm keys set.

The problem is that multiple time shifts will have the same cache key, since the only thing differing them is from_dttm and to_dttm. This results in a false positive cache hit. I fixed it by allowing passing extra cache keys when fetching a dataframe. This way each time comparison will add, eg, time_compare: "1 week" to the cache dict.

[codecov-io]

Codecov Report

Merging #5828 into master will increase coverage by 0.02%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master    #5828      +/-   ##
==========================================
+ Coverage    63.7%   63.73%   +0.02%     
==========================================
  Files         366      368       +2     
  Lines       23143    23220      +77     
  Branches     2599     2600       +1     
==========================================
+ Hits        14744    14800      +56     
- Misses       8384     8405      +21     
  Partials       15       15

Impacted Files	Coverage Δ
superset/viz.py	81.43% <80%> (+0.07%)	⬆️
...ts/src/dashboard/components/gridComponents/Tab.jsx	82.43% <0%> (-3.87%)	⬇️
superset/utils.py	89.02% <0%> (-0.15%)	⬇️
superset/assets/src/explore/components/Control.jsx	38.09% <0%> (ø)	⬆️
superset/data/__init__.py	100% <0%> (ø)	⬆️
superset/assets/src/visualizations/filter_box.jsx	0% <0%> (ø)	⬆️
...uperset/assets/src/visualizations/paired_ttest.jsx
superset/assets/src/visualizations/BigNumber.jsx
.../assets/src/visualizations/BigNumber/BigNumber.jsx	0% <0%> (ø)
.../src/dashboard/components/DeleteComponentModal.jsx	86.95% <0%> (ø)
... and 7 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5eff7a8...79c0203. Read the comment docs.

[mistercrunch]

It'd be nice to add something in the docstring about what extra is used for currently, and could be used for in the future.

[mistercrunch]

Minor comment on improving the cache_key docstring otherwise LGTM