staking miner: less aggresive submissions (#6978)

* staking miner: less aggresive submissions

We have noticed that the staking-miner performs many concurrent RPC calls (more than 256).
Probably because these batch request are getting bigger because the state is growing.

So let's relax this and mine solutions sequentially i.e, mine solution
one solution at the time and not in concurrently.

* add check if self hasn't submitted after mining