Dynamic ping to see if FM is alive

[ArdJonker]

The ping that tracks whether or not FM is alive is in the HA configuration and thus always has the same polling interval. This should be adaptive:

• If the connection is present, polling every half hour is good enough.
• If it is gone it should be every 30 seconds or so.

To achieve this the polling should moved to the flexmeasures_client.py.

[nhoening]

The client has some polling functionality, but I believe not with increasing intervals yet.
Here is an issue asking for that: FlexMeasures/flexmeasures-client#3