Don't ignore Eval.JitAHotSize

Summary: In non-RepoAuthoritative mode, we ignored Eval.JitAHotSize, and
forced it to zero.

This makes it default to zero, while allowing it to be explicitly set.

D1699494 fixes internal configs to preserve the existing behavior.

Reviewed By: @edwinsmith

Differential Revision: D1699534

Signature: t1:1699534:1416750596:85d5177e29e7a21d45577e26a7307a324e8b2cd3