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 Android at https://github.com/owntracks/android/issues/1598

[ckrey]

I think the wipe should be triggered too when

• the userid changes in HTTP or MQTT mode (linked to ACL)
• the clientId changes in MQTT mode (queues in MQTT on both ends are linked to the clientid)
• the `client certificates´ change in MQTT mode (not supported in HTTP mode), because this may be linked to userid/ACL
• the url changes in HTTP mode (may include host, port, userid, password, ...)

[ckrey]

Included in 17.2.5