Allow fallback to default config

[ZenGround0]

Checklist

• [X] This is not brainstorming ideas. If you have an idea you'd like to discuss, please open a new discussion on the lotus forum and select the category as Ideas.
• [X] I have a specific, actionable, and well motivated feature request to propose.

Lotus component

• [X] lotus daemon - chain sync
• [ ] lotus fvm/fevm - Lotus FVM and FEVM interactions
• [ ] lotus miner/worker - sealing
• [ ] lotus miner - proving(WindowPoSt/WinningPoSt)
• [ ] lotus JSON-RPC API
• [ ] lotus message management (mpool)
• [ ] Other

What is the motivation behind this feature request? Is your feature request related to a problem? Please describe.

Last year we set up lotus to never fallback to the default config. The reasoning was that the default config enabled splitstore discard and so default configs are dangerous for users to rely on as they were not yet used to a world with chain store discarding as the default.

It has been a year so I think everyone is pretty well warned. If not we need to find out how to properly warn them now.

Describe the solution you'd like

Remove the default config disallowing option.

Describe alternatives you've considered

We could just keep this forever but it is needlessly inconvenient.

Additional context

This came up because of a bug in a rare code / state path in lotus daemon hit by Zondax that hit this error without benefit.

[rjan90]

Agree on reverting back to the default config now, and setting SpliStore enable as default now that it has been in trasition for over a year.

The disallowing option also seem to cause a issue for Lite-nodes, which needs to explicitly set it as well: https://github.com/filecoin-project/lotus-docs/issues/647.