Use Job repr instead of handle in Client debug log

The debug log are not that useful since they would only output the job
handle.  That is often meaningless:

 DEBUG gear.Client: Job failed; handle: H:127.0.0.1:12222

Instead of the handle, format the string using the job object.
BaseJob.__repr__() let us yield the job name which is much more
informative for the poor human we are:

  DEBUG gear.Client: Job failed; <gear.Job 0x123abcd handle:
  127.0.0.1:12222 name: my_function unique: no_clue>

Change-Id: I06f9203e730061ddd495579836c70f6bbed8f1f3
Merge "Use Job repr instead of handle in Client debug log"