Optimize api.db on cluster

[goshawk-3]

Describe the bug
It turns out that api.db is growing fast in size on any cluster node. We should find a way to clean it up on regular base to avoid wasting disk space.

 1.2G Apr  5 09:53 /opt/dusk/dusk_data/chain/api.db

[goshawk-3]

As per findings, this database stores full provisioners (+stakes) data per each round. We should record provisioners and their stakes only on a change.