Add a workaround to prevent retrieving removed documents from MemDB

[hackerwins]

What this PR does / why we need it:

Add a workaround to prevent retrieving removed documents from MemDB

Removed documents should be filtered out by the query, but somehow it does not work. This is a workaround.

val, err := txn.First(tblDocuments, "project_id_key_removed_at", projectID.String(), key.String(), gotime.Time{})

So this commit adds a workaround to prevent retrieving removed documents from MemDB.

Which issue(s) this PR fixes:

Related to #464

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

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

[codecov]

Codecov Report

Merging #565 (0b7e9a1) into main (7eee551) will decrease coverage by 0.03%.
The diff coverage is 28.57%.

@@            Coverage Diff             @@
##             main     #565      +/-   ##
==========================================
- Coverage   52.27%   52.24%   -0.03%     
==========================================
  Files          67       67              
  Lines        6797     6802       +5     
==========================================
+ Hits         3553     3554       +1     
- Misses       2806     2809       +3     
- Partials      438      439       +1     

Impacted Files	Coverage Δ
server/backend/backend.go	0.00% <0.00%> (ø)
server/backend/database/memory/database.go	53.01% <33.33%> (-0.25%)	⬇️