Feat: caching the aws bucket

[ligangty]

For a single process (uploading|deleting), the bucket will not
change, so we should consider to cache the s3 bucket object for
performance.

[coveralls]

Pull Request Test Coverage Report for Build 1506449947

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 82.777%

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Totals
Change from base Build 1504010005:	0.06%
Covered Lines:	1115
Relevant Lines:	1347

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💛 - Coveralls