Fixes Cache Initialization for HTTP Server

[ArquintL]

ViperHttpServer did not call start() on ViperCoreServer and instead directly called start(active_jobs) on VerificationServer. This resulted in an uninitialized cache and thus in an exception whenever ViperServer tries to store a verification result in the cache after a verification. This PR fixes #111 by making ViperHttpServer call ViperCoreServer.start() and slightly changing the inheritance such that VerificationServerHttp can still configure the server startup (i.e. to bind the HTTP server).

[ArquintL]

bors merge

[bors[bot]]

Build succeeded:

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