Fix race condition in the OPDS 2 parser

[mickael-menu]

Fixed

OPDS

• Fixed race conditions causing ConcurrentModificationException to be thrown when parsing an OPDS 2 feed.

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• Removed the global feed property in the OPDS2Parser, this was the culprit for the ConcurrentModificationException.
• Made all the OPDS models immutable.
  • To avoid messing up with the existing parsing code, I added an internal mutable builder for all the models. When we're ready to revamp the OPDS models and APIs, we can remove them.