Plug the establishConnection() Lisk isolation hole

Summary:
Currently you can still punch through Lisk isolation by calling
establishConnection(), and we do that all over the place. Rename getConnection()
to establishConnection() so that all existing callers are safe, and rename
establishConnection() to establishLiveConnection() so that it's not surprising
when this fails to stub in unit tests.

Not wedded to the name if anyone thinks "establishExternalConnection" or
something is clearer.

Test Plan:
Loaded site, browsed around, ran unit tests.

Reviewed By: aran
Reviewers: aran, tuomaspelkonen, jungejason
CC: aran
Differential Revision: 201