Cleanup on Connection, ConnectionManagerId, ConnectionManager classes part 2

Cleanup on Connection, ConnectionManagerId, and ConnectionManager classes part 2 while I was working at the code there to help IDE:
1. Remove unused imports
2. Remove parentheses in method calls that do not have side affect.
3. Add parentheses in method calls that do have side effect or not simple get to object properties.
4. Change if-else check (via isInstanceOf) for Connection class type with Scala expression for consistency and cleanliness.
5. Remove semicolon
6. Remove extra spaces.
7. Remove redundant return for consistency

Author: Henry Saputra <henry.saputra@gmail.com>

Closes #1157 from hsaputra/cleanup_connection_classes_part2 and squashes the following commits:

4be6906 [Henry Saputra] Fix Spark Scala style for line over 100 chars.
85b24f7 [Henry Saputra] Cleanup on Connection and ConnectionManager classes part 2 while I was working at the code there to help IDE: 1. Remove unused imports 2. Remove parentheses in method calls that do not have side affect. 3. Add parentheses in method calls that do have side effect. 4. Change if-else check (via isInstanceOf) for Connection class type with Scala expression for consitency and cleanliness. 5. Remove semicolon 6. Remove extra spaces.