Make proxies first-class entities

[mhluongo]

Add an abstraction so that proxies can be

• set on session creation
• included in session information
• health checked

[mhluongo]

When you create a new session, you might want a proxy that

• has been used before, but isn't currently
• has never been used before
• is shared between whoever wants it
• is exclusive to the session
• is exclusive to a session tag