statesync: bootstrap node with state obtained out-of-band

[erikgrinaker]

Once state sync ships, we should add support for bootstrapping a node with state that has been obtained by other means as well (e.g. by copying the application state directory) - i.e. instead of fetching and restoring an ABCI snapshot, simply use some existing local state to bootstrap.

This could e.g. be done by adding a use_local_state option to the [statesync] section, which would cause state sync to skip straight to the verification and bootstrap step, possibly with light client verification if necessary.

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[erikgrinaker]

Related to #3713