This adds support for job queries, including (basic) unit-tests.

I've tested this for memory and cpu usage as follows:

- 3600 jobs (live queue):
  - via masterd, default: ~1.1s (masterd: ~60MB ram)
  - via confd,   default: ~1.1s (hconfd:  ~25MB ram)
  - via masterd, id only: ~1.0s (masterd: ~57MB ram)
  - via confd,   id only: ~0.2s (hconfd:  ~15MB ram)

- all jobs (128K in total, around 570MB size on disk):
  - via masterd, default: 1m22s (masterd cpu: 48s), masterd: 1.4G ram
  - via confd,   default: 1m16s (hconfd  cpu: 51s), hconfd:  570MB ram peak
    (peak only right before starting luxi send, hconfd decreases in RSS as
    results are streamed over luxi, back to ~18MB after the send)
  - via masterd, id only:  ~49s (masterd cpu: 45s), masterd: 1.3G ram
  - via confd,   id only:  ~39s (hconfd cpu: 35s),  hconfd:  110MB ram peak
    (right before luxi send, decreasing as results are sent, back to ~14MB
    after the send)

Given this, and that in production it's not likely to have hundreds of
thousand of job files, I believe the implementation is safe from this
point of view.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>