remove global variable `SharedClient`

Because the `staking miner` is using the trait `MinerConfig` and `MinerConfig::solution_weight` doesn't take `&self` it's not possible to feed an RPC client in there without using a global variable.

This requires refactoring the `MinerConfig trait` to take `&self` perhaps it could be possible to adapt this trait accept both `sync` and `async` variants of `solution_weight` as well.