Fix deadlock race condition on compression shutdown

[emersonknapp]

When running the test test_sequential_compression_writer repeatedly, the test sometimes hangs on shutdown, due to a gap in the threading synchronization between the compressor threads and the main thread.

The order of operations is:

• Worker thread N: compressor_thread_fn reads while(compression_is_running_) to be true, so it enters the while block
• Main thread: stop_compressor_threads runs compression_is_running_=false; compressor_condition_.notify_all(), notifying any currently waiting compression threads to wake up, which lets them exit after finding no work to do
  • main thread then calls .join() on all the threads
• Worker thread N: compressor_thread_fn, having entered the while loop successfully, initializes some variables, acquires the compressor_queue_mutex_, and calls compressor_condition_.wait()
• Because the compressor_thread has called wait after the final notify_all, it will never receive a notification, and the .join() call waits forever for this waiting thread to exit

This PR fixes the problem by synchronizing the checks of both exit conditions so that the main thread can't interleave the notifications in a way that causes the deadlock.

NOTE: I am not sure how to add a repeatable regression test for this - the way I can reproduce it is by running the following, which reliably makes it happen quite quickly (within a second). After my change it'll run indefinitely without failure.

build/rosbag2_compression/test_sequential_compression_writer --gtest_repeat=-1 --gtest_throw_on_failure

[zmichaels11]

Do you need to wrap sections where compression_is_running_ is assigned with a lock on the same mutex?

[emersonknapp]

Gist: https://gist.githubusercontent.com/emersonknapp/1d3e63e26efdc9f1596e750454471f19/raw/198dbe7ad4255058e89eefccdfb923da7ff52b7e/ros2.repos
BUILD args: --packages-up-to rosbag2_compression rosbag2_tests
TEST args: --packages-select rosbag2_compression rosbag2_tests
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/7528

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