Remove unnecessary BP lock in ReplicaMap

In HDFS-16429 we make LightWeightResizableGSet to be thread safe, and in HDFS-16511  we change some methods in ReplicaMap to acquire read lock instead of acquiring write lock.

This PR try to remove unnecessary Block_Pool read lock further.

Recently, I performed stress tests on datanodes to measure their read/write operations/second.

Before we removing some lock,  it can only achieve ~2K write ops. After optimizing, it can achieve more than 5K write ops.