Add support for serving static files on background thread

[wch]

This PR adds support for serving files on the background thread that handles I/O.

This minimal example will serve static files out of a subdir of the current directory named content. If there's an index.html file, it will be served if the request is for / (the same is true for subdirectories). If the file isn't found, a minimal 404 page will be served, without hitting the main R thread.

s <- startServer("127.0.0.1", 5000,
  list(
    staticPaths = list("/" = "content")
  )
)

In the next example, it will try to serve requests that have /assets in the URL path from the local assets subdirectory. If the file isn't found, the request will "fall through" -- that is, it will be forwarded to the R call function, like requests normally are with httpuv.

s <- startServer("127.0.0.1", 5000,
  list(
    call = function(req) {
      return(list(
        status = 404,
        body = paste0("404 file not found: ", req$PATH_INFO)
      ))
    },
    staticPaths = list("/assets" = "assets"),
    staticPathOptions = staticPathOptions(
      indexhtml = TRUE,
      fallthrough = TRUE
    )
  )
)

[jcheng5]

Needs documentation

[jcheng5]

Is it correct that there's no return or anything here? We just plow forward with this pFDS?

[wch]

Yeah, the existing behavior was to do that. IIRC, it resulted in an error being thrown in getData(), and this message: Error reading: <filename>. If we fix this, I suggest doing it in a separate PR.

[jcheng5]

Not sure we should fall through here. If a file/directory is missing it's one thing, but if it's present but we fail when reading it, that feels like a true error (and maybe 500 instead of 404).

[wch]

Implemented in f3e487d and 4f499a8.