raft: add simple leader load balancing or sticky leadership

[mmaslankaprv]

When topics are created or cluster is restarted one of the nodes may be a leader for most of the partitions. We need simple load balancing/leadership stickiness to make sure that the whole cluster is evenly loaded and there are no bottlenecks.

[emaxerrno]

we should get @rkruze 's opinion here.

[dotnwat]

we have partition placement now, and we have leadership transfer mechanism. i think we can do something simple like setting a preferred leader bit and then making leadership be a slow controlled process. how do we set this bit? i'm not sure we have to literally set a bit, it kinda seems like if the partition assignments are sufficiently random then each node could independently inspect the assignments and use a heuristic like the first broker in the assignment list is the preferred leader.

[mmaslankaprv]

I have basic implementation of priority based semi-deterministic leader election working in redpanda.