Parallel constraint calculation on SerialMesh too

This will hopefully be a speedup for everybody, but more importantly
it should fix a bug where FEMFunctionBase boundary conditions could
not be used on a SerialMesh on more than one processor.
Comment clarifying a libmesh_error()

I'm choosing to make the SerialMesh behavior less robust here in order
to catch inefficient SerialMesh usage even when that usage isn't
technically incorrect.  We might want to make the error test more
complicated and change this error to a debug-mode warning in the
future.
Only send_receive nodes when we need to

With a serialized mesh sending nodes redundantly is inefficient; with
a synchronized SerialMesh doing so is currently an error.
Merge pull request #356 from libMesh/always_parallel_constraints

Parallel constraint calculation on SerialMesh too