Optimize large file uploads

[falense]

By making use of large file upload features in B2 it should be possible to reduce the memory footprint of B2Fuse to a set maximum memory size. This at the cost of disk space as the entire file must be stored somewhere. Instead of loading the entire file to memory before uploading, chunks could be loaded at a time limiting the maximal memory consumption at any given time.

[leonidasv]

Excited for this feature.