Simplify traits bounds for using Client.

See: https://github.com/CodeChain-io/codechain/pull/970
Everytime we need to add a trait to Client, we have to make a pile of changes to trait bounds on Client. I think we need to simplify this.

Start by reading the linked CodeChain PR 970 and tracing the Client trait bounds it changes. Identify why adding a trait requires edits across those bounds, then define a simpler Client constraint structure and verify that existing Client users still compile.

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