Build thread pool based on memory, not cpus

[robertdfrench]

The limiting factor for concurrency in our case is not cpus (these apps are nearly alway I/O bound), but the amount of memory we'd like to dedicate to portunusd itself. If this can be expressed in the config file (or if a reasonable default can be inferred from the zone's memory allocation) then we can set the threadpool size to S = floor(M/T), where M is the aformentioned memory limit, and T is the per-thread stack size.

[robertdfrench]

See also #14 and #6