Implement document deletion

[cozitive]

What would you like to be added:
Document deletion is one of the essential features. It should be implemented to save resources.

Why is this needed:
Without deletion, accumulated documents may waste storage resources.

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Delivering delete request as a form of Operation
Existing Operation struct can be reused for sending a delete request. If a client send a delete operation, RPC server will deliver it to service server like other normal operations, and Clients/Documents/Packs server in service level will communicate with backend server for deletion.

Broadcasting document deletion to attached clients
Clients attached to a deleted document should know that the document is to be deleted. Packs server must notify it via PushPull process.

Deletion policy
Data cannot be recovered after deleting a document in DB, so deferred deletion is necessary for safety. When a user removes a document, all related data will be pending for deletion. To-be-deleted objects will just be indicated by removed flag, and will be permanently deleted after a pre-defined waiting period by housekeeping service. The waiting period would be determined by an admin.

Data to be deleted:
To completely delete a document, all data related to the document in the server and DB should be eliminated.

struct	DB column	DB related key
DocInfo	documents
ClientDocInfo (in ClientInfo)	clients	documents.${docID}.${*}
ChangeInfo	changes	doc_id
SnapshotInfo	snapshots	doc_id
SyncedSeqInfo	syncedseqs	doc_id

Tasks
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
• 3. Implement housekeeping background process to physically remove document(is_removed = true) and its relevant data.
• 2. Determine whether to include removed documents in ListDocuments API used by Dashboard and CLI(Admin)