Allow reading a TempFile without persisting it

[arnodb]

This PR implements an old suggestion to make TempFile implement AsyncRead.

Technically it is an indirect implementation of AyncRead, and it allows conversion to 'static lifetimes via into_owned.

If eventually one calls into_owned in the middle of reading operations (which is strongly discouraged though), then the reading continues where it was left, not at the beginning of the file.

[SergioBenitez]

I think we should implement anopen() method on TempFile that returns an impl AsyncRead. The concrete type should likely be an Either<File, &'v str> from https://docs.rs/tokio-util/0.6.7/tokio_util/either/enum.Either.html. Then we don't need any manual implementation of AsyncRead nor any intermediary type.

[arnodb]

In the end I don't need it. Two options:

• I reimplement in the suggested way and we manage to merge this PR if you want it
• close and forget the PR

This will be your call.

[SergioBenitez]

For posterity: this was closed because we discovered, on Matrix, that the ultimate intent was to store the file upload on Azure. Using TempFile as an intermediary was inefficient and unnecessary, precisely the reason why TempFile does not and likely will not implement AsyncRead.