Two tests were invariably grinding my system to a halt as it was
applying migrations to an empty sqlite database. This patch reduces
the time of a pytest run from 50 to about 16 seconds on my desktop.
The difference is even more noticable on a slower laptop drive, but
is probably insignificant for all you fancy SSD users.
This adds a new function to history module which returns significant
states. For most domains this is the list of state changes. For the
thermostat domain this also includes attribute changes, so that
changes in the current_temperature are exposed to the graphing layer.
Closes#881
