Show threads in debugger

[reberhardt7]

Currently, the debugger only lists processes. We should list threads if a process has multiple threads, and threads should be independently controllable in the inline debugging controls that appear in the editor.

We can get thread info from gdb; there is already some code tracking threads here. We need to pass this data to debugging.getContainerInfo, embed it in the ContainerInfo object, return it to the client, and update the display to show this info.