Allow all child process' to finish before terminating parent

| Title | Description
| --------------------|------------
| Version | 0.22.1
| Type | Issue
| node | v10.9.0
| Operating System | OSX
| Short Description | Child processes can become orphaned when another child process exits with non-zero
| Detailed description| The first child to exit with non-zero, causes the main bolt process to exit with the same code. In cases where `--parallel` is used, this can leave many orphaned process which keep logging to terminal. In rare cases, orphaned processes can become zombies.

The issue names no source file or test. Start at the --parallel handling and the code that exits the main bolt process when a child returns non-zero. Reproduce with multiple child processes, then verify that all children finish before the parent terminates and that no orphaned or zombie processes remain.

Written by the indexing model from the issue text.