fix `--existing` with non-localhost IP

there's still some cleanup to do with duplicate code in ConsoleApp that should be invoked from ConnectionFileMixin, but this fixes the bug.
add debug statements for kernel clients connecting to channels
add missing 'control' socket to ConsoleApp.load_connection_file
test some connection file utilities

includes regression test for #3977 and some basic coverage for a few utilities.
remove unused imports
Merge pull request #5 from ivanov/existing_ip

remove unused imports
Merge pull request #3978 from minrk/existing_ip

fix `--existing` with non-localhost IP

closes #3977