fix(dashboard): Add caching for dashboard datasets

[suddjian]

SUMMARY

Fixes issues discussed here: #13306 (comment)

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TEST PLAN

Turn on caching, add a breakpoint or log message or other side effect to the cached function, refresh dashboard, verify side effect only happened once.

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #14306 (01e2375) into master (c760030) will increase coverage by 0.20%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master   #14306      +/-   ##
==========================================
+ Coverage   77.01%   77.22%   +0.20%     
==========================================
  Files         954      954              
  Lines       48071    48962     +891     
  Branches     5973     5973              
==========================================
+ Hits        37024    37812     +788     
- Misses      10850    10953     +103     
  Partials      197      197              

Flag	Coverage Δ
hive	80.46% <80.00%> (+0.01%)	⬆️
javascript	72.10% <ø> (ø)
mysql	81.15% <75.00%> (+0.41%)	⬆️
postgres	80.78% <80.00%> (+0.01%)	⬆️
presto	?
python	81.55% <80.00%> (+0.23%)	⬆️
sqlite	80.78% <75.00%> (+0.40%)	⬆️

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

Impacted Files	Coverage Δ
superset/models/dashboard.py	77.41% <75.00%> (+2.41%)	⬆️
superset/dashboards/dao.py	95.45% <100.00%> (-0.13%)	⬇️
superset/db_engine_specs/__init__.py	71.69% <0.00%> (-10.13%)	⬇️
superset/db_engine_specs/presto.py	84.42% <0.00%> (-5.90%)	⬇️
superset/datasets/commands/importers/v1/utils.py	53.57% <0.00%> (-5.34%)	⬇️
superset/db_engine_specs/postgres.py	94.21% <0.00%> (-2.63%)	⬇️
superset/connectors/sqla/models.py	88.61% <0.00%> (-1.46%)	⬇️
superset/config.py	89.85% <0.00%> (-1.18%)	⬇️
superset/models/core.py	88.85% <0.00%> (-0.28%)	⬇️
...erset/dashboards/commands/importers/v1/__init__.py	100.00% <0.00%> (ø)
... and 7 more

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[etr2460]

Looks like there's still some unit test issues to resolve tomorrow

[suddjian]

I suspect it's not kosher to add caching to a function that returns a sqlalchemy object

[etr2460]

this lgtm, I think you'll need to rebase on master to unbreak CI though.

Let's see what the perf impact is here, and i'll have etag stuff soon (as soon as i can actually find time to write code for my job as a software engineer :P )