Issue 6855 - Release all references to Subscription object once removed

To test this:

 1.  Load the extension with the default subscriptions
 2.  Take a heap snapshot in DevTools
 3.  Subscribe to EasyList China at https://adblockplus.org/subscriptions
 4.  Take a second heap snapshot
 5.  Delete EasyList China
 6.  Take a third heap snapshot

When you search for "Subscription" in the heap snapshot, you'll notice that only the second snapshot has one more Subscription object for EasyList China. It is gone again in the third snapshot.

Be sure to run garbage collection manually before taking each heap snapshot.

[Manish Jethani, 2018-08-11 15:28:17]

Patch Set 1

https://codereview.adblockplus.org/29853574/diff/29853575/lib/filterStorage.js
File lib/filterStorage.js (right):

https://codereview.adblockplus.org/29853574/diff/29853575/lib/filterStorage.j...
lib/filterStorage.js:153: subscription.free();
At this point all the other references to this object have been released, except
the one in Subscription.knownSubscriptions.

[Manish Jethani, 2018-08-12 06:51:29]

Patch Set 2: Simplify

Let's do the free method in a separate patch that encapsulated
Subscription.knownSubscriptions properly.

Patch Set 3: Remove duplicate subscription entries from Filter object

A Filter object can contain multiple references to the same Subscription object
when the subscription is loaded from disk.

Patch Set 4: Add tests
Patch Set 5: Add test for duplicate subscriptions

[Manish Jethani, 2018-08-12 07:22:48]

Patch Set 6: Remove Subscription object before emitting "subscription.removed"
event

[Manish Jethani, 2018-08-16 05:34:29]

Patch Set 7: Rebase
Patch Set 8: Improve tests

[hub, 2018-08-16 17:36:25]

LGTM