Snapshot streams

[stonier]

Large PR but conceptually small. This basically emulates the service approach to opening streams on demand as was done for blackboards.

The reason for doing so was to decouple the configuration of the default snapshot stream (which should mostly be used for logging purposes) from the configuration of streams from visualisation clients which would like to enable/disable certain channels on the stream as well as the rate at which it is provided.

Buried in here is a shift to doing activity item streaming properly (not as pre-formatted console text). See also:

• https://github.com/splintered-reality/py_trees/pull/265
• https://github.com/splintered-reality/py_trees_ros_interfaces/pull/8
• https://github.com/splintered-reality/py_trees_ros_interfaces/pull/7
• https://github.com/splintered-reality/py_trees_js/pull/138

Large documentation update to follow.

[stonier]

Ignoring jenkins, it will be broken due to underlying message changes.