Set GOMAXPROCS to number of CPUs

Apparently Go only allows a single thread to be executing by default,
so the increase in performance seen when concurrency was added to
Blueprint was entirely due to scheduling goroutines while the
executing goroutine was blocked in a syscall.  Whoops.

Set GOMAXPROCS to the number of CPUs.  Reduces wall thime by >2x on
one workload.

Change-Id: Ib5fc7761c09a076fc5a1b7f08f5ccf034304477c