Release Riffle version 2.0.0-alpha.47.
Release Riffle version 2.0.0-alpha.47.

 * Upgrade `b-tree` to `2.0.0-alpha.72`.
 * Upgrade `mvcc` to `2.0.0-alpha.3`.
 * Upgrade `b-tree` to `2.0.0-alpha.71`.
 * Upgrade `turnstile` to `6.0.0-alpha.43`.
 * Upgrade `destructible` to `7.0.0-alpha.30`.
 * Upgrade `reciprocate` to `0.0.5`.
 * Upgrade `magazine` to `6.0.0-alpha.5`.
 * Update `LICENSE` for 2021.

Closes #208.

bigeasy/riffle,bigeasy/riffle