Free buffers when overwriting a blob

[ChristopherHogan]

• On a Put, we first need to check if the Bucket already contains the Blob and if so, delete it before replacing it.
• I added a helper function to test_utils.h that required including bucket.h, so I had to make Thallium available to the tests in test/CMakeList.txt.
• In ContainsBlob we can return false if the BlobID is NULL.

[coveralls]

Pull Request Test Coverage Report for Build 501338228

• 8 of 9 (88.89%) changed or added relevant lines in 2 files are covered.
• 5 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.08%) to 80.362%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/metadata_management.cc	4	5	80.0%

Files with Coverage Reduction	New Missed Lines	%
src/metadata_management.cc	2	80.67%
src/stb_ds.h	3	65.59%

Totals
Change from base Build 501302093:	-0.08%
Covered Lines:	4223
Relevant Lines:	5255

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💛 - Coveralls