Improve Synchronizer functionality

This refactors the current Synchronizer into a generic Downloader and a subscription-centered Synchronizer. While at it, some old functionality has been removed and other functionality was added (better redirect handling, versioning of filter data). This also implements the proposals under https://adblockplus.org/blog/simplifying-subscription-downloads-module and https://adblockplus.org/blog/simplifying-subscription-downloads-module.

[Felix Dahlke, 2013-07-25 13:16:02]

LGTM with some nits.

http://codereview.adblockplus.org/11153017/diff/1/lib/downloader.js
File lib/downloader.js (right):

http://codereview.adblockplus.org/11153017/diff/1/lib/downloader.js#newcode175
lib/downloader.js:175: * @param {Downloadable} url  the object to be downloaded
The doc strings are a bit inconsistent. The one above specifies the type inline,
this one uses @param.

http://codereview.adblockplus.org/11153017/diff/1/lib/downloader.js#newcode207
lib/downloader.js:207: let downloadURL = this.getDownloadUrl(downloadable);
Inconsistent camel casing :P

[Thomas Greiner, 2013-07-25 15:21:37]

LGTM
Only a few small suggestions.

http://codereview.adblockplus.org/11153017/diff/1/lib/downloader.js
File lib/downloader.js (right):

http://codereview.adblockplus.org/11153017/diff/1/lib/downloader.js#newcode71
lib/downloader.js:71: maxAbsenseInterval: 1 * MILLIS_IN_DAY,
typo: maxAbsenceInterval

http://codereview.adblockplus.org/11153017/diff/1/lib/downloader.js#newcode204
lib/downloader.js:204: if (downloadable.url in this._downloading)
You could use this.isDownloading(downloadable.url) instead.

http://codereview.adblockplus.org/11153017/diff/1/lib/downloader.js#newcode310
lib/downloader.js:310: this.onDownloadStarted(downloadable);
Seems weird to me that onDownloadStarted is fired before download actually
starts.

[Wladimir Palant, 2013-11-05 06:40:42]

Took me long enough but I finally took care of those nits (you have to use the
View link, not delta).

http://codereview.adblockplus.org/11153017/diff/1/lib/downloader.js
File lib/downloader.js (right):

http://codereview.adblockplus.org/11153017/diff/1/lib/downloader.js#newcode175
lib/downloader.js:175: * @param {Downloadable} url  the object to be downloaded
On 2013/07/25 13:16:02, Felix H. Dahlke wrote:
> The doc strings are a bit inconsistent. The one above specifies the type
inline,
> this one uses @param.

I generally use @param only when there is a need to explain the parameter.
However, inline specification seems fine here if the overall description is
slightly adjusted - and the @param tag gets the name of the parameter wrong
anyway.

http://codereview.adblockplus.org/11153017/diff/1/lib/downloader.js#newcode204
lib/downloader.js:204: if (downloadable.url in this._downloading)
On 2013/07/25 15:21:37, Thomas Greiner wrote:
> You could use this.isDownloading(downloadable.url) instead.

It was hard to convince myself that microoptimizing wasn't worth it here - fixed
:)

http://codereview.adblockplus.org/11153017/diff/1/lib/downloader.js#newcode310
lib/downloader.js:310: this.onDownloadStarted(downloadable);
On 2013/07/25 15:21:37, Thomas Greiner wrote:
> Seems weird to me that onDownloadStarted is fired before download actually
> starts.

Well, nothing really wrong with it (assuming that request.send() won't throw)
but reordering this won't hurt either...

[Felix Dahlke, 2013-11-05 06:46:11]

LGTM