Coil Nailer

• Tested against buster
• Tested against Python 3.7
• Still tested against Python 3.5.
• Still tested against stretch
• Dependencies updated to latest versions
• Improved exception handling of haproxy adder module
• Change haproxy socket connection keep-alive (formerly "timeout") to new
  default: 3600 seconds.
• Updated dependencies with vulnerabilities, not affecting stapled.
• Added the --one-off argument that allows running stapled in one-off mode
  which is a mode that runs staple fetching for all certificates as usual,
  including threading, but stops threads once they are finished with their
  queues and then quits.
• Reduced minimum time to live setting for keeping HAProxy sockets open to 10
  seconds.
• Reduced default time to live setting for keeping HAProxy sockets open to 10
  seconds. This should still be more then enough for all expected conditions
  stapled could run under.
• Add --haproxy-socket-keepalive command line argument.
• Also Note: Python 2 and Jessie support were removed in a previous version.