SwiftNIO 2.20.0

SemVer Minor

• Define NIOBSDSocket namespace (#1471, patch credit to @compnerd)
• Explicit Congestion Notification for UDP (#1473, #1587, #1590, #1591, #1593, #1594, #1596)

SemVer Patch

• Improve performance when removing the last element of a CircularBuffer (#1584, patch credit to @luoxiu)
• Improve pointer hygeine in SocketAddresses.swift (#1588)

Other Changes

• Don't install Jazzy on Xenial (#1597)