multiple context support #668

[wayneparrott]

Adds the ability to support multiple simultaneous rcl contexts, #668.

index.js

• Replaced _nodes array with a Map<context,Array. A list of nodes is maintained for each context provided to rcl.init(context,argv).
• Moved to end of file the import of node.js and time_source.js, the TimeSource property and ShadowNode inheritance function call. This change was made to workaround a circular dependency in rate.js.
• Added context parameter to isShutdown(context=Context.defaultContext())

node.js

• Changed createRate() to return Promise
• Removed the context parameter from createTimer()

rate.js

• Reimplemented RateTimerServer to use public rclnodejs methods.
• Added init(): Promise<void> method which creates internal private context and node

test-init-shutdown.js

• Updated to include a multiple-contexts testcase

test-rate.js

• Updated to use node.createRate(): Promise correctly.

node.d.ts

• Removed context parameter from createTimer() method declaration.

main.ts

• Updated Rate tests for async node.createRate()

[coveralls]

Coverage decreased (-0.8%) to 84.399% when pulling 191aaa356a36628585bf6edc1d4dff7e56c4924c on wayneparrott:multiple_context_support_668 into 3a73bad3c4c335499822ebb9f192e5835a8f8bbd on RobotWebTools:develop.

[wayneparrott]

Note how the appveyor ci build failed during the generate_messages.js script https://ci.appveyor.com/project/minggangw/rclnodejs/builds/33854406

I've seen similar error a few times (seg fault) when installing rclnodejs to a project with foxy debian package installation. I've not had time to investigate the cause yet.

[minggangw]

Paste the error below:

Start JavaScript message generation...
Generation complete.
npm ERR! code ELIFECYCLE
npm ERR! errno 3221225477
npm ERR! rclnodejs@0.14.1 postinstall: `node scripts/generate_messages.js`
npm ERR! Exit status 3221225477
npm ERR! 
npm ERR! Failed at the rclnodejs@0.14.1 postinstall script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.
npm ERR! A complete log of this run can be found in:
npm ERR!     C:\Users\appveyor\AppData\Roaming\npm-cache\_logs\2020-07-01T19_05_42_428Z-debug.log
Command exited with code -1073741819

I've seen this before too, but it seems that the log doesn't give much meaningful information about the cause. I only found this error on Windows (appveyor), not ubuntu (build ROS2 from scratch).

P.S. node scripts/generate_messages.js generates JS messages from .dot template based on the IDL parser.