Implement cartesian plan cache

[methylDragon]

WARNING

 *   This cache does NOT support collision detection!
 *   Plans will be put into and fetched from the cache IGNORING collision.
 *   If your planning scene is expected to change between cache lookups, do NOT
 *   use this cache, fetched plans are likely to result in collision then.
 *
 *   To handle collisions this class will need to hash the planning scene world
 *   msg (after zeroing out std_msgs/Header timestamps and sequences) and do an
 *   appropriate lookup. (very out of scope)

Description

This PR builds on top of:

• https://github.com/osrf/nexus/pull/16

It implements and uses the plan cache for cartesian plans!

Cached plans appear in the move_group_cartesian_plan_cache collection in the db:

[methylDragon]

Will implement changes in a separate PR.