Add RemoveDocument API

[hackerwins]

What this PR does / why we need it:

Add document deletion API.

Document deletion is crucial feature to save resources. Without deletion, accumulated documents and its relevant data will waste storage resources.

The overall document deletion process will look like this:

1. Client sends document delete API(RemoveDocument) with DocIsRemoved = true in request ChangePack
2. In PushPull of RemoveDocument(or other APIs), check DocIsRemoved. And if DocIsRemoved = true, simply set RemovedAt date in docInfo to logically(soft) remove document.
3. Set response ChangePack's DocIsRemoved to true to inform document deletion.
4. After certain period of time(DocumentRemoveDuration), document is physically removed from database.

To successfully implement this API, we need to seperate our tasks and implement them step by step.

• 1. Implement basic document deletion API with simply setting RemovedAt date in docInfo
• 2. Determine whether to include removed documents in ListDocuments API used by Dashboard and CLI(Admin)
• 3. Implement housekeeping background process to physically remove document(is_removed = true) and its relevant data.

Which issue(s) this PR fixes:

Address #464

Special notes for your reviewer:

Discussion about document deletion API in codepair: https://codepair.yorkie.dev/ylvpas

Previous closed PR for implementing document deletion API: #468

WatchDocuments API also needs to find documents by ID, not by key.

Does this PR introduce a user-facing change?:

adds Client.Remove(document), *RemoveDocument API*

Additional documentation:

Checklist:

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

[codecov]

Codecov Report

Merging #484 (540f841) into main (48821a8) will increase coverage by 1.00%.
The diff coverage is 45.16%.

@@            Coverage Diff             @@
##             main     #484      +/-   ##
==========================================
+ Coverage   47.78%   48.79%   +1.00%     
==========================================
  Files          69       69              
  Lines        5826     5962     +136     
==========================================
+ Hits         2784     2909     +125     
+ Misses       2733     2712      -21     
- Partials      309      341      +32     

Impacted Files	Coverage Δ
client/client.go	10.46% <0.00%> (-1.42%)	⬇️
pkg/document/change/pack.go	0.00% <ø> (ø)
pkg/document/document.go	52.11% <0.00%> (-3.95%)	⬇️
server/backend/database/doc_info.go	0.00% <0.00%> (ø)
server/backend/database/mongo/indexes.go	50.00% <ø> (ø)
server/rpc/yorkie_server.go	46.28% <52.23%> (-0.46%)	⬇️
server/backend/database/mongo/client.go	24.90% <54.83%> (+10.52%)	⬆️
pkg/document/internal_document.go	22.03% <66.66%> (ø)
server/backend/database/client_info.go	67.50% <75.00%> (-6.48%)	⬇️
server/backend/database/memory/database.go	52.64% <78.26%> (+0.96%)	⬆️
... and 3 more

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