This includes changes to improve read/write performance.
It is not complete, and will have further changes to add properties and anything else required for MQTT v5.

Conflicts:
	CMakeLists.txt
	ChangeLog.txt
	client/Makefile
	config.mk
	installer/mosquitto.nsi
	installer/mosquitto64.nsi
	lib/mosquitto.h
	lib/mqtt3_protocol.h
	lib/util_mosq.c
	set-version.sh
	snap/snapcraft.yaml
	src/bridge.c
	src/database.c
	src/handle_connack.c
	src/loop.c
	src/persist.c
	test/broker/Makefile

Closes #1196.

Closes #1121.