[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.

Author: zsxwing <zsxwing@gmail.com>

Closes #3634 from zsxwing/SPARK-3154 and squashes the following commits:

0d087ac [zsxwing] Replace ConcurrentHashMap with mutable.HashMap and remove @volatile from 'stopped'
Reorganize the rest 'implicit' methods in SparkContext
Fix the unit test