DirichletBoundaries should be a vector of unique_ptrs

[jwpeterson]

DirichletBoundaries was previously responsible for deleting, but not
allocating, the pointers that it held, which made understanding the
ownership semantics a bit challenging and caused us to have to define
a non-trivial destructor (in dof_map_constraints.C).

[moosebuild]

Job Coverage on 5af4570 wanted to post the following:

Coverage

	babff9	#3270 5af457
	Total	Total	+/-	New
Rate	55.62%	55.62%	-0.00%	66.67%
Hits	44785	44782	-3	4
Misses	35732	35730	-2	2

Diff coverage report

Full coverage report

Warnings

• New new line coverage rate 66.67% is less than the suggested 90.0%

This comment will be updated on new commits.