Adds easy request session-state and corrects cookie domain and path rules

[jhy]

This extends the Connection implementation to support (optional) sessions, which allow request defaults (timeout, proxy, etc) to be set once and then applied to all requests within that session.

Cookies are re-implemented to correctly support path and domain filtering when used within a session. A default in-memory cookie store is used for the session, or a custom implementation (perhaps disk-persistent, or pre-set) can be used instead.

Forms submitted using the FormElement#submit() use the same session that was used to fetch the document and so pass cookies and other defaults appropriately.

The session is multi-thread safe and can execute multiple requests concurrently. If the user accidentally tries to execute the same request object across multiple threads (vs calling Connection#newRequest()), that is detected cleanly and a clear exception is thrown (vs weird blowups in input stream reading, or forcing everything through a synchronized bottleneck.

Examples:

Connection session = Jsoup.newSession()
     .timeout(20 * 1000)
     .userAgent("FooBar 2000");

Document doc1 = session.newRequest()
     .url("https://jsoup.org/").data("ref", "example")
     .get();
Document doc2 = session.newRequest()
     .url("https://en.wikipedia.org/wiki/Main_Page")
     .get();
Connection con3 = session.newRequest();

[jhy]

(Reminder, need to add details to the changelog, particularly a change note for those who may have implemented Connection directly, vs extending HttpConnection)