cttestsrv: fix upstream memory storage API change.

[cpu]

The upstream Trillian project refactored the memory storage provider and since we don't vendor dependencies this broke the cttestsrv.

[coveralls]

Pull Request Test Coverage Report for Build 359

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 67.25%

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Totals
Change from base Build 342:	0.0%
Covered Lines:	768
Relevant Lines:	1142

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💛 - Coveralls