Queue of failed site link requests

[corsacca]

Lets say and contact is being transferred to another D.T instance but that instance is down (doing upgrades) or behind a VPN and not accessible.

Lets catch these failed site link requests and add then to a queue/table.

This will let us:

• retry the request again later with exponential backoff.
• all the other D.T instance to Pull periodically and check if there are request for it that it missed.

For the second option we'll need to build a rest endpoint, check the site link permissions and return the failed requests that were for that D.T instance.

Todo: Decide if this should be a plugin to not make the D.T core even more complicated.