Fetch latest snapshot metadata to determine snapshot creation need

[hyemmie]

What this PR does / why we need it:
To reduce unnecessary data I/O between DB and server, added repository function which only fetches snapshot's metadata.

Which issue(s) this PR fixes:

Fixes #387

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #597 (b44995b) into main (7f39f2f) will decrease coverage by 0.20%.
The diff coverage is 26.08%.

@@            Coverage Diff             @@
##             main     #597      +/-   ##
==========================================
- Coverage   51.45%   51.26%   -0.20%     
==========================================
  Files          66       67       +1     
  Lines        6977     7019      +42     
==========================================
+ Hits         3590     3598       +8     
- Misses       2911     2944      +33     
- Partials      476      477       +1     

Files Changed	Coverage Δ
server/backend/database/snapshot_info.go	0.00% <0.00%> (ø)
server/backend/database/mongo/client.go	38.65% <19.04%> (-0.76%)	⬇️
server/backend/database/memory/database.go	53.00% <50.00%> (-0.14%)	⬇️

[hackerwins]

Thanks for your contribution.