Turn what Link handed back into a connection

Exchanges the one-use public token for the credential this reads with, seals it, and writes the connection. The answer lists every account the owner chose to share, each with the portfolio account it looks like or already feeds, so the next step is pairing. Sending the same public token twice is refused by Plaid rather than making a second connection. Nothing is read and nothing is billed by calling this. Owners only.