Implementation of a buffer release strategy.

* Tests for Release feature.
* Check vector_downward.buf_ before passing to deallocator.
* Assertions.
* Shared test between unique_ptr and GetBufferPointer()
* Unnecessary using directives.
* Reallocate vector if released on clear operation.
* Use allocator attribute.
* Renamed `Release()` to `ReleaseBufferPointer()`
* For consistency with `GetBufferPointer()`
* Updated documentation for ReleaseBuffer.

Change-Id: I108527778e56ae5127abf9e5b1be6b445ad75cb7