[SPARK-3154][STREAMING] Replace ConcurrentHashMap with mutable.HashMap and remove @volatile from 'stopped'

Since `sequenceNumberToProcessor` and `stopped` are both protected by the lock `sequenceNumberToProcessor`, `ConcurrentHashMap` and `volatile` is unnecessary. So this PR updated them accordingly.
`external/flume-sink` doesn't depend on jsr305. However, great to know that we can use `GuardedBy` in Spark.