kernel: Rename in-memory DB option setters and simplify API

[stringintech]

Remove the int parameter from btck_chainstate_manager_options_*_in_memory functions and rename them from "update" to "set". These functions now only set the options to true for enabling in-memory mode. This, combined with explicit default initialization to false, removes ambiguity about what happens when these functions are not called and makes the typical use case (disk-based storage) require no action from the user.

[DrahtBot]

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/33877.

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[DrahtBot]

🚧 At least one of the CI tasks failed.
_{Task Linux->Windows cross, no tests: https://github.com/bitcoin/bitcoin/actions/runs/19390523535/job/55483324598
LLM reason (✨ experimental): Compile error: missing initializer for member coins_error_cb in ChainstateManagerOptions, with warnings treated as errors.}

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[stringintech]

This occurred to me while refactoring the context and chainstate manager options in the Go wrapper, so I opened this PR to get feedback.

cc @TheCharlatan @stickies-v