Improve support for resuming downloads from external files

[Nicklas Nordborg]

The BASE core has support for resuming a file download. This is implemented as calling InputStream.skip(offset) in File.getDownloadStream(offset). In most cases this works well. Local files can simply skip reading the specified number of bytes. But for external files the situation may be different. The default skip implementation simply read the specified number of bytes (which in our case may result in a complete download of the file).

But some protocols have the possibility to specify a start offset (eg FTP) so in this case it would be better to let the remote end handle this.

I think we should add a new interface, ResumableConnectionManager, as a sub-interface to ConnectionManager. The new interface need a single method getInputStream(long offset) and should be implemented by extension that build on protocols that support offsets.

[Nicklas Nordborg]

(In [6493]) Fixes #1820: Improve support for resuming downloads from external files

Added ResumableConnectionManager and implemented this for the built-in HttpConnectionManager.