add LocalFileStorage delay to simulate s3

[PSeitz]

[fulmicoton]

Good idea, but making it a feature is not a good idea.

• --all-features will enable it and no one expects that.
• also it requires recompiling things way too much.

In many places, rust treats feature flags as additive semantically. It is not supposed to change the behavior (we make that mistake in tantivy I think too).

For instnace, if deps A and deps B depends on C with feature flag [feaflag1] and [featflag2] and compatible versions, rustc will compile a single version of C with [featflag1, featflag2].