feat(core.update()): add option to force update writable cores

[Nuhvi]

Helps for cautious restoring of Hypercores on new devices, in combination with truncation.

Admittedly for any eagerly replicated and readily seeded core, waiting for a few minutes should be enough to find the latest state, but still.

[mafintosh]

@Nazeh sorry, this looks good. mind opening on hypercore-protocol/hypercore as we moved to stable?

[Nuhvi]

Done https://github.com/hypercore-protocol/hypercore/pull/302