feat: skip proxy health check if connection is not active

[BruceAko]

Relevant Issue (if applicable)

Fixes #1588

Details

1. Add last_active field for Connection. When Connection.call() is called, last_active is updated to current time.
2. Add max_elapsed_time field for ProxyConfig. Connection is considered to be inactive if the current time to the last_active time exceeds max_elapsed_time.
3. In proxy's health checking thread's loop, if the connection is not active (exceeds max_elapsed_time), this round of health check is skipped.

Types of changes

What types of changes does your PullRequest introduce? Put an x in all the boxes that apply:

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation Update (if none of the other choices apply)

Checklist

Go over all the following points, and put an x in all the boxes that apply.

• [x] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.