Merge pull request #1886 from mozilla/issue-1884-web-channel-test-stall

fix(test): Ensure the web channel tests complete. r=vladikoff
fix(client): The FxDesktop broker no longer sends the `session_status` message on startup.

The call to `session_status` was originally used to determine which page the content server should start at. Firefox now opens content server with the correct page, so this call is no longer necessary.

`session_status` can cause problems. After a logout from Sync, the content server redirects to /signin and Firefox no longer expects a `session_status` message. A channel timeout occurs, and the user is immediately redirected to /500.html.

Bump the channel's DEFAULT_SEND_TIMEOUT_LENGTH_MS to 5000ms

fixes #1883
issue #1830