DirichletBC is MPI blocking

The interface for DirichletBC.apply is MPI blocking. I propose extending the functionality with a non-blocking apply_local method, which does not call assembly-routines on vector or matrices.

This is useful when a set of different DirichletBC are to be applied, with each only living on a subset of processors, e.g.:

bcs = [<list of N DirichletBC instances>]

for bc in bcs:
    bc.apply(A,x) # Waiting on all processes before continuing

An alternative could be:

for bc in bcs:
    bc.apply_local(A,x)

A.apply("insert") # Here is the first block
x.apply("insert")

I'm sketching up some changes to this, and will provide results on a test case on >100 cores to see if there are any improvements.

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