Send Blobs to swap space on GetBuffers failure

[ChristopherHogan]

The old behavior sent Blobs to swap space when CalculatePlacement failed and returned an error when GetBuffers failed. Now we do the opposite. Return an error when CalculatePlacement fails and send the Blob to swap space when GetBuffers fails. I had to add a ForceBlobToSwap function to the tests because triggering a Blob to go to swap from the API would require getting lucky with race conditions.

[coveralls]

Pull Request Test Coverage Report for Build 498786624

• 5 of 7 (71.43%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 80.446%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/api/bucket.cc	1	2	50.0%
src/buffer_pool.cc	4	5	80.0%

Files with Coverage Reduction	New Missed Lines	%
src/data_placement_engine.cc	1	95.73%

Totals
Change from base Build 498757956:	-0.03%
Covered Lines:	4225
Relevant Lines:	5252

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💛 - Coveralls