client: revamp Subscribe API

[ijsong]

We have not yet devoted enough attention to the Subscribe API. Additionally, known bugs can cause panic when subscribing to a wide range. The code base is also complicated, making maintaining, identifying, and resolving issues challenging. Furthermore, many function names are difficult to infer their functionalities, but also side-effect on their tasks.
I believe it's time for a revamp.

This issue can be resolved by several smaller issues:

• client: fix panic caused by too large channel size in pkg/varlog.subscribedLogEntriesQueue #653
• client: improve code readability of Subscribe API in pkg/varlog #654
• client: redesign user API of Subscribe and SubscribeTo in pkg/varlog #655