Timing issues when starting plugin hosts

[tsmaeder]

Bug Description:

While investigating eclipse-che/che#16502, I noticed that some messages from plugins get lost when starting up: connections to plugin hosts are started as soon as the the connection module for a Theia front end is constructed on the back end. At this time, the connection back to the front end may not be set up yet. When this is the case, messages from plugin hosts can be silently ignored, for example https://github.com/eclipse-theia/theia/blob/master/packages/plugin-ext/src/hosted/node/hosted-plugin-process.ts#L156
In particular, "setClient" is called after connection to the plugin host: since the plugin host starts operating upon connection, it may send message that are lost.

Steps to Reproduce:

There is really no reproducible case for this: however, we seem to see some intermittent issues in Che, where startup of plugin hosts are slower (and we have more of them).

Additional Information

• Operating System: n/a
• Theia Version: 1.4