Companion PR for #7039: grandpa-rpc dont share subscription manager, only executor (#1687)

* grandpa-rpc: dont share subscription manager, only executor

* node/rpc: merge imports

* node: apply style fixes from code review

* "Update Substrate"

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>
Co-authored-by: parity-processbot <>
Co-authored-by: André Silva <andrerfosilva@gmail.com>
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