Use Flask stream when writing data to storage

[glpatcern]

Supersedes https://github.com/cs3org/wopiserver/pull/138.

We use the flask write stream to spare an in-memory buffer copy (the actual request is still processed in one go as it's a POST).

The writefile() signature was changed to accept either a byte[] buffer or a stream as content, and the size as an extra parameter (-1 would mean that the function must compute it assuming content is a buffer).

To be tested for all interfaces:

• [x] xroot
• [x] cs3
• [x] local (CI will be sufficient)

[wkloucek]

@micbar @DeepDiver1975 can you please test this with oCIS / the CS3 interface or schedule it to the testing to the team?

[DeepDiver1975]

time wise out of scope on my end - sorry :cry:

[glpatcern]

With the help of @aduffeck we successfully tested this with all backends, therefore I'll proceed with merging it.