Add sessionId and reconnection

[sadym-chromium]

Currently,

1. The session with the Browser instance are closed whenever the connection is closed.
2. There is no way to reconnect if the connection is dropped.
3. WPT tests need a /session endpoint on the server.

Consider to implement reconnection like in classical WebDriver.

Open questions:

• how to close the session?

Related topics:

• https://github.com/w3c/webdriver-bidi/issues/97
• https://github.com/w3c/webdriver-bidi/issues/103
• https://github.com/w3c/webdriver-bidi/issues/104