[pilight] Add option to disable background discovery for a given pilight bridge thing...

[pilight] Add option to disable background discovery for a given pilight bridge thing (#15986)

* [pilight] Add option to disable background discovery for a given pilight bridge thing

Previously the background discovery updated periodically all channels of all
devices regardless whether they have sent an update or not.  This behavior
makes it impossible to decide whether a device is still alive by observing
channel updates sent by a device itself.  Especially for devices running on
battery, it is important to know, if it still sends updates.

Create ExecutorService during initialize

---------

Signed-off-by: Stefan Roellin <stefan@roellin-baumann.ch>