Combine both TimeInterval types into single type

[RobAtticus]

Query generation and Cassandra's query running both used a type
called TimeInterval that did roughly the same thing. This change
combines the two into one type that can be used from the utils
package in internal/. This improves code reuse and keeps the two
representations in sync, and also increases the testability of
the code.

[codecov-io]

Codecov Report

Merging #75 into master will increase coverage by 0.47%.
The diff coverage is 60.55%.

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
+ Coverage   57.13%   57.61%   +0.47%     
==========================================
  Files          76       77       +1     
  Lines        3719     3770      +51     
==========================================
+ Hits         2125     2172      +47     
- Misses       1570     1573       +3     
- Partials       24       25       +1

Impacted Files	Coverage Δ
...d/tsbs_generate_queries/databases/influx/devops.go	32% <0%> (ø)	⬆️
...bs_generate_queries/databases/clickhouse/devops.go	9.94% <0%> (ø)	⬆️
...d/tsbs_generate_queries/databases/siridb/devops.go	26.92% <0%> (ø)	⬆️
cmd/tsbs_generate_queries/uses/devops/common.go	96.77% <100%> (-0.06%)	⬇️
...s_generate_queries/databases/timescaledb/devops.go	98.63% <100%> (+0.01%)	⬆️
...sbs_generate_queries/databases/cassandra/devops.go	24.09% <27.27%> (-0.6%)	⬇️
internal/utils/time_interval.go	95.83% <95.83%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3cfaae0...3ce1a1f. Read the comment docs.

[antekresic]

Superfluous the?

Also, I'm from the camp which avoids over-commenting, which in this case, IMO, this would be. Sometimes unnecessary comments can be just as bad as missing comments. I do understand its a personal thing mostly.

Scratch that, its exported. I'm blind.

[antekresic]

Changes look good!

[RobAtticus]

Hey @antekresic , asking for a re-review because I added MustRandWindow for cleaner caller code. So this handles the panic issue by doing it in the library (we can eventually remove this in favor of bubbling up the errors).

I also rewrote the tests to use the t.Run trick I learned in your other PR :)

[antekresic]

Nice changes, looking good!