Experimental for Linux, change thread priority to execute timer.

[fujitatomoya]

Experimental for Linux, this can fix ros2/ros2#1035

[fujitatomoya]

result with this PR,

• rmw_fastrtps

# ros2 launch composition composition_demo.launch.py
...
[component_container_mt-1] [INFO] [1601975831.752306501] [talker]: Publishing: 'Hello World: 1'
[component_container_mt-1] [INFO] [1601975831.752824676] [listener]: I heard: [Hello World: 1]
[component_container_mt-1] [INFO] [1601975832.751991412] [talker]: Publishing: 'Hello World: 2'
[component_container_mt-1] [INFO] [1601975832.752274885] [listener]: I heard: [Hello World: 2]
[component_container_mt-1] [INFO] [1601975833.752379498] [talker]: Publishing: 'Hello World: 3'
[component_container_mt-1] [INFO] [1601975833.752655100] [listener]: I heard: [Hello World: 3]
[component_container_mt-1] [INFO] [1601975834.752039205] [talker]: Publishing: 'Hello World: 4'
[component_container_mt-1] [INFO] [1601975834.752408299] [listener]: I heard: [Hello World: 4]
[component_container_mt-1] [INFO] [1601975835.752011603] [talker]: Publishing: 'Hello World: 5'
[component_container_mt-1] [INFO] [1601975835.752307486] [listener]: I heard: [Hello World: 5]
[component_container_mt-1] [INFO] [1601975836.752418197] [talker]: Publishing: 'Hello World: 6'
[component_container_mt-1] [INFO] [1601975836.752799341] [listener]: I heard: [Hello World: 6]
[component_container_mt-1] [INFO] [1601975837.752058776] [talker]: Publishing: 'Hello World: 7'
[component_container_mt-1] [INFO] [1601975837.752340178] [listener]: I heard: [Hello World: 7]
[component_container_mt-1] [INFO] [1601975838.752041790] [talker]: Publishing: 'Hello World: 8'
[component_container_mt-1] [INFO] [1601975838.752337113] [listener]: I heard: [Hello World: 8]
[component_container_mt-1] [INFO] [1601975839.752072457] [talker]: Publishing: 'Hello World: 9'
[component_container_mt-1] [INFO] [1601975839.752397260] [listener]: I heard: [Hello World: 9]
[component_container_mt-1] [INFO] [1601975840.752017258] [talker]: Publishing: 'Hello World: 10'
[component_container_mt-1] [INFO] [1601975840.752366232] [listener]: I heard: [Hello World: 10]

• rmw_cyclonedds

# ros2 launch composition composition_demo.launch.py
...
[component_container_mt-1] [INFO] [1601975951.680059475] [talker]: Publishing: 'Hello World: 1'
[component_container_mt-1] [INFO] [1601975951.680459358] [listener]: I heard: [Hello World: 1]
[component_container_mt-1] [INFO] [1601975952.680332440] [talker]: Publishing: 'Hello World: 2'
[component_container_mt-1] [INFO] [1601975952.680609332] [listener]: I heard: [Hello World: 2]
[component_container_mt-1] [INFO] [1601975953.680392413] [talker]: Publishing: 'Hello World: 3'
[component_container_mt-1] [INFO] [1601975953.680725646] [listener]: I heard: [Hello World: 3]
[component_container_mt-1] [INFO] [1601975954.680405710] [talker]: Publishing: 'Hello World: 4'
[component_container_mt-1] [INFO] [1601975954.680738803] [listener]: I heard: [Hello World: 4]
[component_container_mt-1] [INFO] [1601975955.680375269] [talker]: Publishing: 'Hello World: 5'
[component_container_mt-1] [INFO] [1601975955.680731282] [listener]: I heard: [Hello World: 5]
[component_container_mt-1] [INFO] [1601975956.679945772] [talker]: Publishing: 'Hello World: 6'
[component_container_mt-1] [INFO] [1601975956.680305675] [listener]: I heard: [Hello World: 6]
[component_container_mt-1] [INFO] [1601975957.680425607] [talker]: Publishing: 'Hello World: 7'
[component_container_mt-1] [INFO] [1601975957.680804461] [listener]: I heard: [Hello World: 7]
[component_container_mt-1] [INFO] [1601975958.680441078] [talker]: Publishing: 'Hello World: 8'
[component_container_mt-1] [INFO] [1601975958.680848182] [listener]: I heard: [Hello World: 8]
[component_container_mt-1] [INFO] [1601975959.679987377] [talker]: Publishing: 'Hello World: 9'
[component_container_mt-1] [INFO] [1601975959.680262910] [listener]: I heard: [Hello World: 9]
[component_container_mt-1] [INFO] [1601975960.680468220] [talker]: Publishing: 'Hello World: 10'
[component_container_mt-1] [INFO] [1601975960.680724972] [listener]: I heard: [Hello World: 10]

[fujitatomoya]

closing this in favor of #1516