Changed random eviction cache to accounts only

[SirTyson]

Description

This change adds a configurable memory based cap to the LiveBucketList random eviction cache. It also modifies the cache such that only accounts are stored.

Following initial testing, the random eviction cache improved TX flooding performance, but had virtually no effect on apply time. This makes sense, since apply time already leverages the ltx cache and only makes at most one disk read per ledger entry. TX flooding on the other hand has no such queue and constantly re reads entries off disk. Given that we already have an ltx cache for apply time, it makes sense to only cache the entry types that are used by transaction flooding (i.e. ACCOUNT).

My initial default setting is to cache 3 GB. Currently accounts take up about 2.5 GB in the BucketList, so this allows us to cache all accounts with some room for growth, and 3 GB seems like a modest increase in RAM requirements for validators.

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