resume interrupted chunked downloads

Leverage the new chunked remotes to automatically resume downloads.
Sort of like rsync, although of course not as efficient since this
needs to start at a chunk boundry.

But, unlike rsync, this method will work for S3, WebDAV, external
special remotes, etc, etc. Only directory special remotes so far,
but many more soon!

This implementation will also properly handle starting a download
from one remote, interrupting, and resuming from another one, and so on.

(Resuming interrupted chunked uploads is similarly doable, although
slightly more expensive.)

This commit was sponsored by Thomas Djärv.

Utility/Metered.hs

@@ -88,7 +88,7 @@ meteredWriteFile meterupdate f b = withBinaryFile f WriteMode $ \h ->
 
 {- Applies an offset to a MeterUpdate. This can be useful when
  - performing a sequence of actions, such as multiple meteredWriteFiles,
- - that all update a common meter progressively.
+ - that all update a common meter progressively. Or when resuming.
  -}
 offsetMeterUpdate :: MeterUpdate -> BytesProcessed -> MeterUpdate
 offsetMeterUpdate base offset = \n -> base (offset `addBytesProcessed` n)