Added ability to query message store by ObjectId (python only for now).

Also added some little bits of logging.
Now returning from query srv
Added ObjectID into meta on query return
Made id query return single element.
Returning id correctly from service call.
Merge pull request #28 from hawesie/hydro-devel

Changes for strands_executive
Changed to get the deserialisation length from the vector length.

This removes the bug where variable length types were incorrectly deserialised. Thanks to @nilsbore for reporting the bug.
Merge pull request #31 from hawesie/hydro-devel

This fixes my problem, merging!
Adding an initial rostest for the message store proxy.
Adds a service to delete message by ID
Adding delete function to MessageStoreProxy and using it in unittest.
Merge pull request #34 from cburbridge/hydro-devel

Adding an initial rostest for the message store proxy.
Merge remote-tracking branch 'upstream/hydro-devel' into cherry_picks

Conflicts:
	ros_datacentre_cpp_client/include/ros_datacentre/message_store.h