Kernel/SDK: write multiple chunks as part of store_write

Context

Previously, manually writing chunks to storage (particularly in the case of 'appending to a value') required manually managing the chunks that were being written. This MR simplifies this significantly.

Checklist

• 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).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR