BP-65: Implement load balance for select bookie

[TakaHiR07]

Motivation

Implement bookie load balance to avoid bookie high write latency problem.

Master issue: #4247

Changes

The proposed changes involves:

1. Implement BaseMetricMonitor in bookie server, which would collect bookie load information periodically
2. bookie client continue to use getBookieInfo restApi to acquire load information from each bookie.
3. modify the implementation of RackawareEnsemblePlacementPolicyImpl, support select ensemble by LoadWeightBasedPlacement. Since LoadWeightBasedPlacement is an enhancement of DiskWeightBasedPlacement, it would cover the DiskWeightBasedPlacement if feature enable.

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