engine: enable mimalloc abandoned_page_reset option and call mi_collect() during enginePrepareLoadBoard()

I think changing the default value here is the only way to ensure the option is set at process initialization time, before control is passed to main(). It looks like the "official" way to enable this is to set some environmental variables before spawning the process, but I don't think that solution is really appropriate for production builds.

engine: add mimalloc to take care of allocations that smmalloc doesn't handle that would otherwise fall through to the system allocator