Release B-tree version 2.0.0-alpha.67.
Release B-tree version 2.0.0-alpha.67.

 * Some concerns that turned into documentation. See #735.
 * Test housekeeping race. Closes #734.
 * Optionally externalize part serialization. Closes #733.
 * Fix page read race condition. Closes #732.

Closes #736.

bigeasy/strata,bigeasy/strata