Mix: Add connection maintenance

[youngjoon-lee]

1. What does this PR implement?

For spam protection and resiliency, the Tier 1 design contains the Connection Maintenance, which measures frequencies of several incoming messages, and close/open connections to maintain the reasonable quality of delivery.
How frequencies are measured is defined in the Frequencies Measurement. How to close/open connections is defined in here.

This PR implements all mentioned above.

Plus, this implementation provides an option to disable it. Ideally, all nodes should enable connection maintenance. However, we are not sure what much frequency we should expect yet. If we set the threshold too high or small, the protocol would close too many healthy connections than intended. The estimation is not simple even though we have the persistent transmission, because of the immediate forwarding.
I will run tests soon with multiple (virtual) nodes to measure the actual frequencies, so that we can provide the recommended parameters to set for the connection maintenance.

2. Does the code have enough context to be clearly understood?

Yes

3. Who are the specification authors and who is accountable for this PR?

@youngjoon-lee @madxor

4. Is the specification accurate and complete?

Yes

5. Does the implementation introduce changes in the specification?

No

Checklist

Warning

Do not merge the PR if any of the following is missing:

• 1. Description added.
• 2. Context and links to Specification document(s) added.
• 3. Main contact(s) (developers and specification authors) added
• 4. Implementation and Specification are 100% in sync including changes. This is critical.
• 5. Link PR to a specific milestone.

[danielSanchezQ]

i think it is much clearer now! Thanks for the extra effort!