add --sequential to valkey-benchmark (for populating entire keyspace)

[SoftlyRaining]

I needed a way to systematically populate the keyspace before running valkey-benchmark tests like get, so I added a --sequential option that modifies the behavior of -r so that the replacements are sequential numbers in the keyspace instead of random numbers.

Example:

# populate keys by sequentially setting them
./valkey-benchmark -d 512 --sequential -r 3000 -n 3000 -c 650 -P 4 --threads 64 -t set -q

# benchmark get with 100% hit rate
./valkey-benchmark -r 3000 -n 250000 -c 650 -P 4 --threads 64 -t get

[codecov]

Codecov Report

Attention: Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.

Project coverage is 71.04%. Comparing base (bcd2f95) to head (f7bf025).

Files with missing lines	Patch %	Lines
src/valkey-benchmark.c	91.30%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1839      +/-   ##
============================================
+ Coverage     70.97%   71.04%   +0.06%     
============================================
  Files           123      123              
  Lines         65665    65672       +7     
============================================
+ Hits          46608    46655      +47     
+ Misses        19057    19017      -40     

Files with missing lines	Coverage Δ
src/valkey-benchmark.c	62.11% <91.30%> (+0.30%)	⬆️

... and 14 files with indirect coverage changes

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[enjoy-binbin]

LGTM, seems like a good idea.

[madolson]

@valkey-io/core-team Explicitly pinging yall for votes, as this is a new API on one of our tools.