What

Change the interface of the RPC for fetching pages. We now use the slot_id to identify the slot instead of the commitment.

Why

To give us the possibility of indexing slots and shards by slot_id instead of commitment in the DAL node store.

Document the interface of any function added or modified (see the coding guidelines)
Document any change to the user interface, including configuration parameters (see node configuration)
Provide automatic testing (see the testing guide).
For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
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