v0.1.0-alpha.6

There are a number of API changes, fixes, performance boosts, and log improvements included in this release.

As we're still in pre-release, we don't have a formal changelist of everything included, but here are a few things worth noting:

• Breaking: an API change on a stream from shutdown() to an async close(), which returns after pending writes are acknowledged by the recipient
• Breaking: require &mut self in AsyncUdpSocket trait methods
• Faster transfers and fewer timeouts in high-throughput scenarios that cause UDP packet loss, by pulling packets off the read queue as quickly as possible
• Clearer log messages about different possible scenarios
• Much more...