Make mapUpdateLoop() indicator variables to be thread-safe

[AlexeyMerzlyakov]

This is a elaboration of https://github.com/ros-planning/navigation2/pull/3307/files#r1038041946 comment for #3307.
Costmap2DROS contains one std::thread parallel execution, which calls mapUpdateLoop() routine.

As stated in a C standard docs in a "5.1.2.4 Multi-threaded executions and data races"

The execution of a program contains a data race if it contains two conflicting actions in
different threads, at least one of which is not atomic, and neither happens before the
other. Any such data race results in undefined behavior.

So, all write-write and write-read parallel operations should be protected by mutex, or std::atomic in case if it is the variable.

The mapUpdateLoop() thread contains the following boolean indicator variables appeared to be thread-unsafe:

• stop_updates_: being read in Costmap2DROS::updateMap() loop called from std::thread; and written in parallel in the Costmap2DROS::start() routine called from main thread.
• initialized_: being read in Costmap2DROS::start(), which belongs to main thread; and written in parallel by Costmap2DROS::updateMap() from std::thread.
• stopped_: to be thread-unsafe since Ensure that plugin initialization to be called before updating routines #3307, according to the https://github.com/ros-planning/navigation2/pull/3307/files#r1038041946 comment

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses	#3307
Primary OS tested on	Ubuntu 20.04
Robotic platform tested on	colcon test --packages-select nav2_costmap_2d

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Description of contribution in a few bullet points

• Protected all bool indicator variables in mapUpdateLoop() thread from unsafe usage.

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Future work that may be required in bullet points

• It would be cool to check all places with the std::thread utilization in Nav2, and find all shared variables (bool, int, string, vectors, etc...) and make them to be thread-safe, if they not.

For Maintainers:

• Check that any new parameters added are updated in navigation.ros.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists

[mergify]

@AlexeyMerzlyakov, please properly fill in PR template in the future. @SteveMacenski, use this instead.

• Check that any new parameters added are updated in navigation.ros.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists

[SteveMacenski]

God I hate all the stuff going on in this file. I welcome the day I can delete all this and do a rewrite