We have various problems with reallocating the array of Capabilities,
due to threads in waitForReturnCapability that are already holding a
pointer to a Capability.

Rather than add more locking to make this safer, I decided it would be
easier to ensure that we never move the Capabilities at all.  The
capabilities array is now an array of pointers to Capabaility.  There
are extra indirections, but it rarely matters - we don't often access
Capabilities via the array, normally we already have a pointer to
one.  I ran the parallel benchmarks and didn't see any difference.