Technical design of account calibration

[Keith-CY]

This issue comes from #464 where a workflow of how to calibrate accounts between frontend and backend was roughly proposed.

sequenceDiagram
    participant Frontend
	participant Backend
    loop
    Frontend ->> Frontend: H = Hash(Address set)
    Frontend ->> Backend: get_balance(H)
    Backend ->> Frontend: H is not found
    Frontend ->> Backend: subscribe_address_set(address set)
    Backend ->> Backend: H = Hash(Address set)
    Frontend ->> Backend: get_balance(H)
    Backend ->> Frontend: balance of address set H
    Frontend ->> Frontend: address set grows to address set'
    Frontend ->> Frontend: H' = Hash(Address set')
    end

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The account calibration is required by the account model of UTXO/Cell model.

To exploit UTXO/Cell model, an account is typically a set of addresses, e.g., a HD-wallet, so each UTXO can be assigned to a debutante address and the identity of recipient is hard to predicate.

It should be a general fundamental of DApp built on CKB, so address calibration should be delivered within Kuai natively.

Here is how it will be supported in Kuai:

---

In the backend, an account should be located by a hash(address set) and mapped to a set of addresses. The mapping is ephemeral and will be discarded if it's not accessed for a long time. So redis is quite suitable for this case.

The following features are required:

1. cache multiple entries, each one can be mapped by a hash key;
2. each entry has a ttl and will be removed when timeout. The ttl should be refreshed on accessing;
3. the size of entries is limited, and the one that remains the least ttl will be removed once a new entry is inserted;

maxmemory 1G
maxmemory-policy volatile-ttl # remove stale entries first
SET accounts {hash(address_set)} addresses {address_list]} EX 600 # insert an entry with 600s lifetime
GET accounts {hash(address_set}) # get an entry
EXPIRE accounts {hash(address_set)} # refresh ttl

It's quite simple, so I think a singleton could hold all logic.