Improve api/activity/recent_items performance

[escherize]

On stats, it is ~500ms but hopefully should be ~200ms.

e.g.: /api/activity/recent_views 200 452.1 ms (6 DB calls)

We should profile this endpoint to see what is actually taking so long, but there are some possible avenues that we suspect:

1. The 4 entity queries (:card, :dataset, :collection, and :table) are done sequentially, but could be done in parallel

2. Removing runtime checks (costs: could return incorrect data in certain situations).

3. Allow filtering via params (either count of items, or count per model, or just specify which models to grab)

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• profile endpoint and figure out what is expensive

[escherize]

Before any optimization, this is where we are on my m1 macbook pro:

on master:

Evaluation count : 4440 in 60 samples of 74 calls.
             Execution time mean : 15.107747 ms
    Execution time std-deviation : 1.086454 ms
   Execution time lower quantile : 13.752945 ms ( 2.5%)
   Execution time upper quantile : 17.359551 ms (97.5%)
                   Overhead used : 2.389407 ns

Found 1 outliers in 60 samples (1.6667 %)
	low-severe	 1 (1.6667 %)
 Variance from outliers : 53.4768 % Variance is severely inflated by outliers

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Here's a flamegraph with recent-views highlighted. Seems like the time is indeed dominated by the big 4 (or 5) queries. I will see if parallelizing them makes a dent now.

[escherize]

I suspect parallelizing the 4 big queries on the left would speed this up. On my small test data size, however, the overhead made it 10% slower.

[paoliniluis]

I have a strong belief that a massive improvement might come if we go with #16909