Ensure that `pygls.Server` uses the event loop it is given

[alcarney]

Description (e.g. "Related to ...", etc.)

Before this commit pygls would override the event loop for the current thread even when given an existing loop to use.

Now pygls will rely on asyncio.new_event_loop to create the appropriate event loop for the platform and only call it when loop=None

This commit also introduces the _own_loop flag which is used to make sure pygls does not close an event loop it does not own.

Related: #334

Code review checklist (for code reviewer to complete)

• [x] Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• [x] Title summarizes what is changing
• [x] Commit messages are meaningful (see this for details)
• [/] Tests have been included and/or updated, as appropriate
• [/] Docstrings have been included and/or updated, as appropriate
• [/] Standalone docs have been updated accordingly
• [/] CONTRIBUTORS.md was updated, as appropriate
• [x] Changelog has been updated, as needed (see CHANGELOG.md)