New Features

• Implement Server.start_serving() and related APIs.
  (by @jlaine in 7a4f00a)

• Make Server an asynchronous context manager.
  (by @jlaine in d6c67e7 for #221)

Performance

• Start using high-performance uv_udp_t handle. This
  should result in a significantly faster UDP support.
  (by @1st1 in c2b65bc)

• Make address validation faster in udp.sendto().
  (by @1st1 in 46c5e9e)

Build

• Upgrade to libuv v1.28.0.
  This is a minimum requirement now.
  (by @1st1 in c2b65bc)

• Use manylinux-2010. This means we no longer provide
  32bit builds for Linux.
  (by @1st1 in 3174b7d)