Performance improvement for first - avoids unnecessary pagination.

[simonprickett]

first() was running the ft.search query to get the first item, then also paginating the whole dataset unnecessarily. This PR addresses that by setting exhaust_results=False whenever first calls query.execute(). This stops the pagination. Closes #218 and adds a test for first() using HashModel.

[codecov-commenter]

Codecov Report

Merging #219 (129ef95) into main (c0fa1be) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #219   +/-   ##
=======================================
  Coverage   77.25%   77.25%           
=======================================
  Files          12       12           
  Lines        1165     1165           
=======================================
  Hits          900      900           
  Misses        265      265           

Flag	Coverage Δ
unit	77.25% <100.00%> (ø)

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Impacted Files	Coverage Δ
aredis_om/model/model.py	86.27% <100.00%> (ø)

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