Store subscription map in etcd

[hackerwins]

What this PR does / why we need it:

Store subscription map in etcd.

Which issue(s) this PR fixes:

Fixes #183

Special notes for your reviewer:

We need to merge #213 first.

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 #214 (7f48909) into main (93abec2) will decrease coverage by 0.72%.
The diff coverage is 1.85%.

@@            Coverage Diff             @@
##             main     #214      +/-   ##
==========================================
- Coverage   58.57%   57.85%   -0.73%     
==========================================
  Files          44       44              
  Lines        3684     3732      +48     
==========================================
+ Hits         2158     2159       +1     
- Misses       1327     1374      +47     
  Partials      199      199              

Impacted Files	Coverage Δ
yorkie/backend/sync/etcd/pubsub.go	0.00% <0.00%> (ø)
yorkie/backend/sync/memory/coordinator.go	0.00% <0.00%> (ø)
yorkie/rpc/yorkie_server.go	45.74% <0.00%> (-0.38%)	⬇️
yorkie/backend/sync/memory/pubsub.go	94.36% <100.00%> (+0.08%)	⬆️

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[dc7303]

LGTM 👍

[dc7303]

@hackerwins In the future, in cluster mode, do you plan to manage subscriptions only in etcd instead of memory.pubsub? If you could explain a little more why this work is necessary, it would be good for reference in other work in the future.

[hackerwins]

I'm still thinking about the following.

1. SubscriptionMap must remain as it has to manage the channels with the clients.
2. It seems to be better to move Metadata to MongoDB for metadata updates: Improve Peer Awareness's metadata to be updatable #153.