- simply use a basic {String, String} struct
- fix MQTT_TOPIC_TIME vs. MQTT_TOPIC_DATETIME
- get rid of unused chaining and indexes

- shrink utils source file, move heartbeat and boot management into system
- improvise with 'heartbeat' functionality. include scheduler implementation that will
manage the per-module heartbeat callbacks with individual 'mode' and
'interval' settings. current ones are mqtt (including relays, lights, thermostat), debug and
influxdb. preserve heartbeat NONE, ONCE and REPEAT, REPEAT_STATUS is effectively a hbReport & status bit.
- mqtt heartbeat is managed through mqttHeartbeat() callbacks
- tweak mqtt callbacks to use lists instead of the vector, slighly reducing the size of the .bin
- update WebUI, include report setting and update hbMode values
- make sure general.h settings include new heartbeat,
move constant definitions outside of the header
- correctly include dependencies through the .cpp, avoid leaking
internal details.
- as a side-effect, base headers are no longer included recursively

- general conversion from .ino modules into a separate .cpp files
- clean-up internal headers, place libraries into .h. guard .cpp with _SUPPORT flags 
- fix some instances of shared variables instead of public methods
- tweak build system to still build a single source file via os environment variable ESPURNA_BUILD_SINGLE_SOURCE

* mqtt: enum state

* mqtt: async client buffer with MQTT_MAX_PACKET_SIZE

* mqtt: rework debug messages

* mqtt: MQTT_BUFFER_MAX_SIZE

* mqtt/test: debug log for async callback

* mqtt/test: don't log things we don't handle

* button: fix typo

* Config: remove prototypes.h

* fixup order

* word

* add missing

* fix warning while still using .ino

* bad at c/p;