The special remote `directory` support files split in chunks, other special remotes do not.

Support for chunks is useful, for example, to be able to upload large files over slow, unreliable connections or to minimize the amount of data to be sent when only part of a big file has been changed.

Couldn't the code used to split, checksum and reconstruct the files in the `directory` remote be used also in all the other special remotes?

> [[done]]; nearly all special remotes support chunking now, and the ones
> that don't omit it for their own reasons, for example bup is not sped up
> by using chunks, and glacier needs some additional work to support them.
> --[[Joey]]
