Add Server field to witchcraft.InitInfo

[Brad Moylan]

Some initialization functions require a way of launching a goroutine that may eventually call Shutdown on the server. Currently this is being worked around by creating the initFn as a variable after constructing the server, using a reference to the server inside the anonymous function, then setting the initFn on the server before starting it.

Instead, we could provide a Server field on InitInfo. It should have descriptive documentation explaining the state(s) the server can be in when initfn is called.