Enable background eviction by default

[SirTyson]

Description

Resolves #4415

Drops "experimental" from background eviction flag and enables it by default.

Checklist

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