Reduce dependencies

[stephank]

This removes some dependencies:

• Remove HTTP/2 support. I don't believe this is useful for the few outgoing requests we make. The h2 crate has seen only a couple of advisories from what I remember, but it is also ~18K SLOC talking to untrusted hosts, that doesn't bring us significant benefit.
• Vendored the sd_notify bits we use. This also fixes the STOPPING notification, which I believe never worked. (The env var was unset by the READY notification.)
• Replaced the matches crate with std::matches.
• Removed our usage of lazy_static. Still used by dependencies.