Consider caching file attributes for static content

On simple static workload
```
static("static") {
file("js/script.js")
}
```
caching result of `file.isFile` invocation saves up to 30% of throughput (depending on file size).
For static content it can be cached either forever or in timely manner (e.g. via Guava's `Suppliers#memoizeWithExpiration`).
The rest of attributes (byte content or last modified time required for `LocalFileContent#`) can probably be cached as well

Start by tracing the static("static") file("js/script.js") path and the file.isFile invocation, then inspect LocalFileContent initialization and its byte-content and last-modified requirements. Measure the simple static workload before and after caching; done means improved throughput without stale or incorrect file attributes.

Written by the indexing model from the issue text.