Implement an in-memory DB of db interface

[hackerwins]

What would you like to be added:

Implement an in-memory DB of db interface.

Currently, MongoDB is required to run Yorkie to store its data. Persistent storage is required for normal use cases, but running MongoDB for simple testing is cumbersome. Since many users are new to Yorkie, we need to provide an in-memory DB so that users can test Yorkie simply.

https://yorkie.dev/docs/main/agent#running-agent

Packages related to DB are as follows:

• yorkie/yorkie/backend/db package defines interfaces of methods and types that Yorkie access when using DB.
• yorkie/yorkie/backend/db/mongo package is a implementation MongoDB version of the db package.
• yorkie/yorkie/backend/db/memory package is a implementation in-memory version of the db package.

If the user passes MongoDB configuration by passing flags related to MongoDB, Yorkie uses MongoDB to store its data. Otherwise, Yorkie stores its data to in-memory DB like flags related to ETCD.

Why is this needed:

• Users can simply run Yorkie without MongoDB to test it.
• We can speed up tests in CI.