* Rename nav2_tasks

Now that ROS2 actions have been implementation and the temporary tasks implementation
is obsolete, all of the task-related code has been removed from the repo. What's
left in the nav2_tasks directory is Behavior Tree related code. So, this PR renames
nav2_tasks to nav2_behavior_tree. This new directory will contain any of our
domain-specific nodes and BT-related utility classes.

* Fix a couple more cpplint/uncrustify issues

* Fix a rebase error

Signed-off-by: vinnamkim <vinnam.kim@gmail.com>

Extended the SimpleActionServer interface and improved a few other things:
* Added additional checks to make sure we only keep the latest preempt request in the queue.
* Action server users no longer interact directly with the goal handles. 
* Provided additional functions for aborting, canceling, etc.
* Updated bt_navigator, motion_primitives, dwb_controller, navfn_planner to use the new interface.

Refactored the DWB controller main loop.
* Added checks for handling pending preemptions after the controller has succeeded or failed.

* Addressing bt_navigator crash when goal pose is
provided before the node is ready.

* Added activate() and deactivate() to action server.

Longer time intervals can't be converted to smaller intervals.

* Adding a dockerfile that builds all of ROS2 in a single workspace.

This catches missing dependencies in the package.xml files.

* Adding some comments on choice of docker starting image.

* Adding pre-release checklist.

* Removing unneeded build code

* Update to dashing branches.

* Change name to better reflect reality

* Make full_build dockerfile configurable

* ARG before FROM doesn't propogate to lines after FROM

* Updating docs and addressing review content

* More doc fixes.

* Fixing review comments

* brought back amcl's map topic functionality

* removed amcl ready check

* fix linter

* changed the default to not use map topic

* removed unused code

* factored out duplicated code and fixed a bug

* changing default to map topic

shutdown nodes in reverse order of bringup

* Precise control of the contained node to improve shutdown reliability

* Add a couple TODO comments about further reducing code, eliminating duplication

* Add params for rolling window, no static layer

* Add a transform tolerance duration

* Add support for time differences

* Fixing a typo

* Missed a rebase issue.

* Make system test params match bringup params so this change gets tested

* Fix system test to use bringup params file