Implement setting/clearing thermostat schedule

[ebaauw]

• Implement POST and DELETE for /sensors/id/config/schedule/Wbbb, see New Device Support Request - eCozy Smart Heating Thermostat #2393;
• Added sanity checks when reading the serialised schedule from the database, to prevent segmentation faults, see New Device Support Request - eCozy Smart Heating Thermostat #2393;
• Fixed handling of utc attributes for ZHATime, now that core returns number of seconds since Zigbee epoch (2000/01/01) instead of since Unix epoch (1970/01/01);
• Stop trying to poll the gateway device. Gateway now no longer issues route requests for itself.

[manup]

Stop trying to poll the gateway device. Gateway now no longer issues route requests for itself (NWK 0x0000).

Nice, saw that the other day in a sniffer log. Still a bit strange why it caused route requests as other requests for ZDP stuff also work for the gateway.

[ebaauw]

WTF, I get compile errors on Stretch

$ qmake -v
QMake version 3.0
Using Qt version 5.7.1 in /usr/lib/arm-linux-gnueabihf
$ uname -a
Linux pi1 4.19.66-v7+ #1253 SMP Thu Aug 15 11:49:46 BST 2019 armv7l GNU/Linux

No issues on Buster

$ qmake -v
QMake version 3.1
Using Qt version 5.11.3 in /usr/lib/arm-linux-gnueabihf
$ uname -a
Linux pi6 5.4.51-v7l+ #1333 SMP Mon Aug 10 16:51:40 BST 2020 armv7l GNU/Linux

[ebaauw]

I think this PR is good to go. @manup @SwoopX could you please have a look?

I still see one "poll node" message for the gateway device, shortly after startup. I don't know where it comes from.