[HOTFIX][CORE] fix a concurrence issue in NewAccumulator

## What changes were proposed in this pull request?

`AccumulatorContext` is not thread-safe, that's why all of its methods are synchronized. However, there is one exception: the `AccumulatorContext.originals`. `NewAccumulator` use it to check if it's registered, which is wrong as it's not synchronized.

This PR mark `AccumulatorContext.originals` as `private` and now all access to `AccumulatorContext` is synchronized.

## How was this patch tested?

I verified it locally. To be safe, we can let jenkins test it many times to make sure this problem is gone.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #12773 from cloud-fan/debug.