Bugfix for attaching to graphs

This release primarily fixes #4, and involved a complete refactor of the GraphServer.

• Significantly reduces technical debt in GraphServer, Publisher and Subscriber
• Fixes #4
• Adds a test for attaching to running graphs
• Adds a __version__ attribute and configures packaging to use that attribute for version
• Version bump to 3.1.2
• Exposes GraphServer and GraphContext at the top level of ezmsg.core
• Strong version checking between connecting clients and GraphServer

Initial Public Release

Hopefully Github Actions is set up to actually publish all extensions!

[sorry if this has been spamming you!]