Feature request: Add setUploading() method

[gmaclennan]

This is a request to add a method to control whether a core is uploading, e.g. whether it will respond to "wants" from other peers by sending data. I think the undocumented setActive method does this for downloading, but not for uploading.

Use case 1: A peer might have limited upstream bandwidth, and only want to download data.

Use case 2: A peer might want to get an updated length from a peer before deciding to enable uploading and downloading.

Related: #555

I think #305 can be closed, superseded by this issue and #555.