B-tree v2.0.0-alpha.37

• Use new Journalist.drain() in shutdown. See #662.
• Implement Strata.drain(), Cursor 100% coverage. Closes #662.
• Remove ghosts. Closes #580. Closes #660.
• Calculate and return heft on insert/remove. Closes #661.
• Notes on counts and Merkle. See #10.

Closes #663.