[db] Provide in-memory db for api layer

[Liuhaai]

What would you like to be added

Read cache in API ./api/read_cache.go should be integrated into db layer

Why is this needed

How important you think this is for IoTeX

• [] must have
• [] should have
• [x] nice to have

Additional information

Let us know any background or context that would help us better understand the request (for example the particular use-case that prompted this request)

[raullenchai]

We have no intention to implement