add an experimental websockets api between client and server

[devoncarew]

• add an experimental websockets api between client and server
• listen for it at the /ws endpoint
• add the ability to call the version command over this channel
• have a compile-time flag to enable the client using websockets (with the flag disabled for this PR)

Note that the websocket connection is long-lived (we'd need to adjust the backend configuration, but we can set requests timeouts to up to 60m). Before actually enabling this, we'd want to:

• better understand resource behavior on the backend (we'd need one analysis server and one front-end server instance running per websocket connection)
• tear down those resources when a websocket connection terminates
• have the client switch to using a websocket connection for all commands
• have the client transparently reconnect when a websocket connection terminates
• understand the new number of continuous server instances required (would we end up using 10x the number of backend instances?)

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[johnpryan]

LGTM, a few minor comments

[johnpryan]

I would probably still prefer to create this object in the AppServices constructor (or not, depending on the useWebsockets flag) so that this code is easier to migrate in the future

[devoncarew]

OK, sounds good - creating a websocket connection eagerly, but only if we have the compile time useWebsockets const set.