Expire CredentialsRef in a proper time to reduce memory usage

[zhouyifan279]

1. Describe the feature

This is the a sub-task under umbrella issue #913.

In sub-task #1007, <user, CredentialsRef> pairs are cached in a Map. Credentials are periodically renewed and will never be removed.
If a lot of inactive users' credentials are cached, both memory usage and credentials renewal load may become unacceptable.
So we should implement a logic to expire cached <user, CredentialsRef> pairs to erase this risk.

2. Motivation

3. Describe the solution

4. Additional context

[lightning-L]

Hi, I'd like to try to implement this.

[ulysses-you]

@lightning-L assigned, thank you