Better behavior when BucketListDB account cache disabled

[SirTyson]

Description

This improves behavior when the account cache is disabled in BucketListDB. Previously, BUCKETLIST_DB_MEMORY_FOR_CACHING == 0 was still valid, but we would still create a random eviction cache object that had no capacity. This change makes is such that we don't construct the cache at all in this case. I've also added an explicit test.

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document