Make sure LocalFile handles get closed

[garrettjstevens]

As described in #488, I was getting this error when loading data on the demo server:

    Error: importFeatures failed — 422 Unprocessable Entity 
    ({"message":"Error: EMFILE: too many open files, open 
    '/data/uploads/05dc2a73502099956dce87e417ed886c'","error":"Unprocessable 
    Entity","statusCode":422})

It turns out #488 did not fix this, so after some more digging I found the solution in this PR. It turns out we have to be a bit careful about making sure we close filehandles when using LocalFile so that we don't have a lot of opened files.

[cmdcolin]

not sure if it's relevant but I was considering modifying the way LocalFile works in GMOD/generic-filehandle#114 to not maintain an open filehandle. this way it would close and re-open for each read. while it could result in slightly less performance (benchmark could be made) it would reduce the need to meticulously manage the open/closing of the file (it is implicitly opened....explicitly closed as in this PR)

[cmdcolin]

(that particular change could be made without the other Uint8Buffer related API adjustment that is proposed)

[garrettjstevens]

That could also work. One thing I considered was using an LRU cache of filhandles instead of closing each one, but I didn't try it since, same as you mentioned, I wasn't sure what the performance benefit would be for that added complexity.