Multithreaded levelling, bug fixes

What's Changed

Levelling calculation is now multi-threaded

• Threaded levelling with adjusted algorithm by @springfall2008 in #1122
• Fixes to threaded levels, disable fast mode history by @springfall2008 in #1125

Bug fixes

• Backed out faster get_history after some people had corrupted data with this method
• Car charging smart, make as early as possible for cheapest slots by @springfall2008 in #1123
• Fix to crash with zero discharge windows by @springfall2008 in #1124

Full Changelog: v7.19.4...v7.19.5