job-manager: add flux job submit --debug flag

[garlick]

Problem: it's difficult to know what's going on inside the job manager. For example, a pending alloc request is "retracted" (flag indicating that it's outstanding cleared) when the scheduler is unloaded, and then the alloc request is resent when the scheduler is loaded again.

One idea is to add a debug flag at job submission. If set, the job manager could emit verbose debug eventlog entries for the job. The key events that drive the job state machine are already in there. This would surround it with some further context about what's happening to use as a trace for testing, or a debugging tool.