Adds indexeddb support to servo. At the moment heed is being used as the
backend, although this can be swapped out by implementing `KvsEngine`.
This PR adds a thread + a thread pool for Indexeddb related operations.
Also `database_access_task_source` is added for Indexeddb related
operations.
This is a partial rewrite of #25214. (Reopened due to branching issue)
Fixes#6963
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Signed-off-by: Ashwin Naren <arihant2math@gmail.com>
Signed-off-by: Josh Matthews <josh@joshmatthews.net>
Co-authored-by: Rasmus Viitanen <rasviitanen@gmail.com>
Co-authored-by: Josh Matthews <josh@joshmatthews.net>
