Prevent races by blocking on SDK builds

If we don't block on SDK builds, then the riot-web build fails due to half-built dependencies. This needs to be done at two levels: the js-sdk because it is used by both the react-sdk and riot-web, and at the react-sdk because riot-web needs it. This means our build process is synchronous for js -> react -> riot, at least for the initial build. 

This does increase the startup time, particularly because the file watch timer is at 5 seconds. The timer is used to detect a storm of file changes in the underlying SDKs and give the build process some room to compile larger files if needed. 

The file watcher is accompanied by a "canary signal file" to prevent the build-blocking script from unblocking too early. Both the js and react SDKs build when `npm install` is run, so we ensure that we only listen for the `npm start` build for each SDK.

This is all done at the riot level instead of at the individual SDK levels (where we could use a canary file to signal up the stack) because:
* babel (used by the js-sdk) doesn't really provide an "end up build" signal
* webpack is a bit of a nightmare to get it to behave at times
* this blocking approach is really only applicable to riot-web, although may be useful to some other projects.

Hopefully that all makes sense.

package-lock.json

@@ -384,6 +384,12 @@
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       "dev": true
     },
+    "async-lock": {
+      "version": "1.1.3",
+      "resolved": "https://registry.npmjs.org/async-lock/-/async-lock-1.1.3.tgz",
+      "integrity": "sha512-nxlfFLGfCJ1r7p9zhR5OuL6jYkDd9P7FqSitfLji+C1NdyhCz4+rWW3kiPiyPASHhN7VlsKEvRWWbnME9lYngw==",
+      "dev": true
+    },
     "asynckit": {
       "version": "0.4.0",
       "resolved": "https://registry.npmjs.org/asynckit/-/asynckit-0.4.0.tgz",