[Proposal] refactor the LoadBalancer to support more rebalance strategies

[vagetablechicken]

Is your feature request related to a problem? Please describe.
We want to add more strategies for load balance, actually for rebalance. And the new strategies may want to execute a concrete move of a replica from one server to another, a move can be described as {tablet, from_be, dest_be}. Thus, the rebalance class should support to delete the specified replica(in from_be).
So we need to define a new base class for rebalance, the origin rebalance strategy could work on it as well.

Describe the solution you'd like
The new base class, named Rebalancer.
class methods:

• selectAlternativeTablets: use a strategy to determine which tablet to rebalance.

• createBalanceTask: try to create a clone task for rebalance.

• getToDeleteReplicaId: needed by the new priority "replica is src replica of rebalance" in handleRedundantReplica.The origin rebalance doesn't need to override getToDeleteReplicaId. The move-based strategies should override it.

• updateLoadStatistic: the rebalancer may have cache, so it should be stateful. Add a method to update the load statistic.

The origin LoadBalancer, which will named as BeLoadRebalancer, just need to extend from Rebalancer without any logic change.