Fix unexpected split calls

This MR aims to avoid calls to Context.split on contexts using the always indexing strategy. This prevents context corruptions due to such unexpected calls.

Manually testing the MR

Run an sandboxed node with TEZOS_CONTEXT="indexing-strategy=always" and bake a few blocks: it should fail at some point. Restart it without the env. var. and with the patch. It should work.

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