allow server resurrection after repltakeover

Today a dragonfly master shutdown upon successful repltkeover flow.

@ashotland suggests that instead dragonfly will stop accepting connections on the main port and will close all the existing connections.

It will still be possible to revert this operation via the admin port by allowing the connection to connect to the main port.

Start by tracing the repltakeover flow and the main and admin port connection handling described in the issue. Confirm the current shutdown behavior; done means the server stops accepting main-port connections, closes existing connections, and can be reverted through the admin port.

Written by the indexing model from the issue text.