rkt status: implement status

Simple querying of container status by uuid is provided.

The pid of the container's stage1, an exited boolean, and per-imageid
exit status codes are printed when available.

Container exit synchronization occurs when --wait is specified.

Examples:
 Already exited but not yet garbage collected container:

   $ rkt status --uuid 32e5c3d4-c851-4c46-bddc-b52c46449523
   pid=11219
   exited=true
   sha256-f9124faae79eb859b27581d9673a2d29a8e394a2685088753264dea276dd7571=0
   $

 Already exited and "marked" for garbage collection by rkt gc (same as above):

   $ rkt status --uuid 32e5c3d4-c851-4c46-bddc-b52c46449523
   pid=11219
   exited=true
   sha256-f9124faae79eb859b27581d9673a2d29a8e394a2685088753264dea276dd7571=0
   $

 Already exited and actively being discarded by rkt gc (treated as gone):

   $ bin/rkt status --uuid 874d5c50-ee5e-4cb9-9b30-c95778398e43
   Unable to query status: "874d5c50-ee5e-4cb9-9b30-c95778398e43" is being removed
   $

 Currently executing container:

   $ rkt status --uuid 8b06b7af-7d53-4723-bc0c-fe6e47463fcb
   pid=12227
   exited=false
   $

 Waiting on potentially executing container to exit for exit status:

   $ time bin/rkt status --uuid ec5df2c6-b5d9-495d-aea0-c2cf5da706c2 --wait
   pid=12319
   exited=true
   sha256-f9124faae79eb859b27581d9673a2d29a8e394a2685088753264dea276dd7571=0

   real    0m21.470s  <<< note delay before returning
   user    0m0.028s
   sys     0m0.004s
   $

   $ time bin/rkt status --uuid ec5df2c6-b5d9-495d-aea0-c2cf5da706c2 --wait
   pid=12319
   exited=true
   sha256-f9124faae79eb859b27581d9673a2d29a8e394a2685088753264dea276dd7571=0

   real    0m0.033s  <<< note zero delay when already exited
   user    0m0.024s
   sys     0m0.008s
   $