v0.2.5: Many fixes and Alpine/Containerization support

First release since August! It's been a while! This release contains several fixes:

• Dependencies were managed better / upgraded across the board
• We now conform to "Rust 2021".
• Error messages were better described when errors connecting to zerotier-one were encountered (thanks @laduke for the patch!)
• ZEROTIER_CENTRAL_INSTANCE can now specify non-canonical instances of zerotier central (thanks @glimberg for the request!)

Additionally, thanks to the diligence of @altano we have a pretty comfortable Alpine Linux solution now, as well as quality containerization techniques with it:

• The README goes into how to run your own containerized zeronsd that runs independently of the host (but still shares a networking stack). This allows you to better manage your zeronsd instances; it is not a security improvement over running on the host due to how the authtoken is handled.
• Alpine Linux on the host machine is now a first class product with openrc support in for zeronsd supervise and zeronsd unsupervise. These scripts were provided by @altano and adapted into the source code.

To pull the docker images: docker pull zerotier/zeronsd:alpine-v0.2.5 or zerotier/zeronsd:alpine-latest.

A gentle reminder we also have Homebrew, Windows, .deb and .rpm packages as well as through cargo. These are the preferred and supported versions of zeronsd. Check out the README for more information!

NOTE: the windows and mac os x builds contain post-tag compiler fixes for situations where the code would not build, but these do not alter functionality under these platforms.