Remove redundant dict_index calculations

[nadav-levanoni]

We need to start making use of the new WithDictIndex APIs which allow us to reuse the dict_index calculation (avoid over-calling getKeySlot for no good reason).

In this PR I optimized lookupKey so it now calls getKeySlot to reuse the dict_index two additional times.

The benefit here is reduced if the slot is cached in the client, which is generally the case.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.72%. Comparing base (771918e) to head (e940f02).

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1205      +/-   ##
============================================
+ Coverage     70.71%   70.72%   +0.01%     
============================================
  Files           114      114              
  Lines         63090    63091       +1     
============================================
+ Hits          44616    44624       +8     
+ Misses        18474    18467       -7     

Files with missing lines	Coverage Δ
src/db.c	88.86% <100.00%> (+0.09%)	⬆️

... and 10 files with indirect coverage changes

[nadav-levanoni]

Will add a follow-up PR for keys and scan commands. The impact will be greater because they do not cache the slot in the client.

[xbasel]

lgtm

[nadav-levanoni]

I added the keysCommand optimization here. I had a cursory look at the scan command, and it might be trickier to find an optimization there.