Manual seal delayed finalize (#13999)

* up

* up

* added test

* remove unncessary dep

* cargo fmt

* cargo fmt

* up

* Update client/consensus/manual-seal/src/lib.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* fix test

* cargo fmt

* added docs

* updated doc

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Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: parity-processbot <>