Fully reset configuration when changing mode and/or host/port

[jpmens]

As briefly discussed internally, I suggest an enhancement which will reset our apps' configuration and internal database on change of host/port for a particular mode or on change of mode (iOS does the latter already).

The reasoning is that if I'm hosted on a particular broker but change to a different friend's broker, say, there's no reason to keep a list of locations I've received in the past, nor is there a reason to see prior friends, as they'll likely not be on the new broker / server.

In particular we should wipe:

• current list of friends (contacts received)
• accumulated and not yet transmitted locations (which might even be destined for an incorrect topic)

Tracked on iOS at owntracks/ios#771

[jpmens]

Additional ground for a wipe

[growse]

There's currently no code that wipes the outstanding queue, so that'll take some thought. Clearing the contacts should be straightforward.