[SPARK-10001] Consolidate Signaling and SignalLogger.

## What changes were proposed in this pull request?
This is a follow-up to #12557, with the following changes:

1. Fixes some of the style issues.
2. Merges Signaling and SignalLogger into a new class called SignalUtils. It was pretty confusing to have Signaling and Signal in one file, and it was also confusing to have two classes named Signaling and one called the other.
3. Made logging registration idempotent.

## How was this patch tested?
N/A.

Author: Reynold Xin <rxin@databricks.com>

Closes #12605 from rxin/SPARK-10001.