Update GraphCache for services & clients along with remaining graph introspection methods

[Yadunund]

This PR builds on top of #86 but with changes from #87. Reviewing might be easier if we merge #87 in and rebase #86.

Changes introduced:

• Liveliness tokens are put/deleted for service and client entities and GraphCache is updated with this information.
• Implement all remaining graph introspection functions.
• Fixes liveliness tokens when node or topic names have / in the middle. Eg /add_two_ints_server/get_type_description. Previously such names would get appended to the liveliness token without any substitution which could cause the parsing and reconstruction of liveliness::Entity to have incorrect values for types and qos. Now a SLASH_REPLACEMENT char is introduced and such names and mangled and demangled to correctly transmit and reconstruct graph information.

To Test:

# terminal 1
  ros2 run demo_nodes_cpp add_two_ints_server

# terminal 2
ros2 service list

# result
/add_two_ints_server/set_parameters
/add_two_ints_server/set_parameters_atomically
/add_two_ints
/add_two_ints_server/get_type_description
/add_two_ints_server/get_parameter_types
/add_two_ints_server/list_parameters
/add_two_ints_server/get_parameters
/add_two_ints_server/describe_parameters

ros2 node info /add_two_ints

# result 
/add_two_ints_server
  Subscribers:
    /parameter_events: rcl_interfaces/msg/ParameterEvent
  Publishers:
    /rosout: rcl_interfaces/msg/Log
    /parameter_events: rcl_interfaces/msg/ParameterEvent
  Service Servers:
    /add_two_ints_server/get_type_description: type_description_interfaces/srv/GetTypeDescription
    /add_two_ints: example_interfaces/srv/AddTwoInts
    /add_two_ints_server/get_parameter_types: rcl_interfaces/srv/GetParameterTypes
    /add_two_ints_server/list_parameters: rcl_interfaces/srv/ListParameters
    /add_two_ints_server/get_parameters: rcl_interfaces/srv/GetParameters
    /add_two_ints_server/describe_parameters: rcl_interfaces/srv/DescribeParameters
    /add_two_ints_server/set_parameters_atomically: rcl_interfaces/srv/SetParametersAtomically
    /add_two_ints_server/set_parameters: rcl_interfaces/srv/SetParameters
  Service Clients:

  Action Servers:

  Action Clients:

Known issue (currently being investigated)

Starting 3 or more nodes causes the liveliness get query at node initialization to hang indefinitely.

• Fixed by 689d102

[clalancette]

I've left a few things to think about, but overall I think this is pretty good. I have a whole bunch of fixes to go on top of this, so hopefully we can get this cleaned up and merged soon.