feat(limit-*): add custom reject-message for traffic control

[okaybase]

What this PR does / why we need it:

Add custom reject-message for traffic control, service provider can tell the caller what happens.

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[tzssangglass]

@spacewander I think we need to unify the rejected_code and rejected_msg attributes of the limit-req, limit-conn and limit-count.

At present they look somewhat incongruous.