Introduce MongoDB sharding rules to Client-wide collections

[sejongk]

What this PR does / why we need it:
This commit introduces MongoDB sharding rules to clients and client-wide collections (e.g. syncedSeqs).

Some fields continue to use client_id instead of (client_key, client_id):

• actor_id in the changes and syncedSeqs collections
• owner in the documents collection
• publisher in DocEvent of the pubsub package

Currently, Yorkie utilizes the objectID in MongoDB as a client_id.
According to the official MongoDB post, the objectID is composed of 12 bytes (TimeStamp(4 bytes) + MachineId(3 bytes) + ProcessId(2 bytes) + Counter(3 bytes)), making it unlikely to be duplicated in practical use cases.
Furthermore, the chances of actor_id and publisher in a single document being duplicated are even lower.
Considering Yorkie isn't currently expected to exceed the practical use cases, maintaining the client_id reference key in the aforementioned concerns seems reasonable.

In addition, the client_key currently doesn't have any use case, including the owner in the documents collection. It seems possible to remove the client_key field from clients and client-wide collections.
Then, I believe we can use client_id as a shard key instead (with an unique constraint), eliminating the risk of duplicate IDs in client_id.

Moreover, there are alternatives for handling duplicate client_ids:

• Introduce an UUID generator into the Yorkie cluster
• Use actor_id as a combination of client_key and client_id
  • This may increase the size of CRDT metadata and the size of a Document snapshot.

Which issue(s) this PR fixes:

Addresses #673

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

Attention: 31 lines in your changes are missing coverage. Please review.

Comparison is base (7b3df9d) 50.73% compared to head (f07f121) 50.73%.

Files	Patch %	Lines
client/client.go	0.00%	8 Missing ⚠️
server/backend/database/mongo/client.go	60.00%	8 Missing ⚠️
server/backend/database/client_info.go	0.00%	5 Missing ⚠️
server/rpc/yorkie_server.go	82.75%	4 Missing and 1 partial ⚠️
api/types/resource_ref_key.go	0.00%	2 Missing ⚠️
server/backend/database/memory/database.go	80.00%	0 Missing and 2 partials ⚠️
server/backend/housekeeping/housekeeping.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #768   +/-   ##
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  Coverage   50.73%   50.73%           
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  Files          70       70           
  Lines       10234    10274   +40     
=======================================
+ Hits         5192     5213   +21     
- Misses       4517     4536   +19     
  Partials      525      525           

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