Make OpenFile filelike

[martindurant]

@rabernat : a pickleable OpenFile in-a-context looks something like this, see what you think. Now entering with returns the OpenFile itself, which is now really file-like. It exists so that a chain of file-likes (compression, text...) can be closed in sequence at the right time. None of this affects what you get when you do a fs.open(), which objects are often pickleable (http, s3, local...), but this is not guaranteed.

Note that it breaks a couple of previous pickle tests, I will need to see whether the assumptions there are valid or not. Particularly, what should it mean to unpickle a "w"-mode file - naively, a new file gets opened with mode "w", invalidating the previous copy and truncating the file.

[rabernat]

Thanks a lot for working on this Martin!

I'm not sure that the with context manager behavior is relevant for Pangeo Forge and pangeo-forge/pangeo-forge-recipes#391 (comment). The conclusion we reached in #579 (comment) (see also google/xarray-beam#49 (comment)) was that, in order to achieve the desired serializability, we should NOT use a context manager. Consequently, you will find no context managers in the Pangeo Forge file opener code, but rather just fs.open() style.

[martindurant]

This change removes OpenFile.open.<locals>.close, which was used for pickling, and is a consequence of calling OpenFile.open (as opposed to filesystem.open), and showed up in the error message, so probably it is fixed here.