Add ClickHouseBackend(client_fn=...) option to keep chains separate

[michaelosthege]

Closes #14

[codecov-commenter]

Codecov Report

Merging #22 (a21a90c) into main (291c489) will increase coverage by 0.10%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
+ Coverage   92.91%   93.01%   +0.10%     
==========================================
  Files           8        8              
  Lines         536      544       +8     
==========================================
+ Hits          498      506       +8     
  Misses         38       38              

Impacted Files	Coverage Δ
mcbackend/backends/clickhouse.py	94.11% <93.33%> (+0.29%)	⬆️

---

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 291c489...a21a90c. Read the comment docs.