Multitasking is a requirement

[ZipCPU]

Current behavior

Only one proof runs at a time

Expected behavior

All of the CPUs on a machine should be enabled by default for proofs. For those engines with known numbers of threads, allocate tasks until the machine is full. Always (by default) run at full speed.

In a later revision, measure the current CPU usage, and only start a new task if 1) no tasks are running and one has been requested, or 2) the system load is low enough that another task will not peg it.