Server: Worker refactor

[jaymell]

• Add webhook-http-client, which is based on hyper, enabling several
  critical improvements:
  • Default filtering of endpoints that resolve to private
    address space, with ability to add CIDR exceptions as needed
  • Case-sensitive HTTP1 headers -- although HTTP spec states that
    headers are not case-sensitive, in practice many widely
    deployed web-servers expect to support case-sensitive headers.
    This requires a fork of hyper, which currently has but does
    not expose support for case-sensitive headers.
  • Assurance that the host header appears first in outgoing HTTP requests.
    Although HTTP spec does not prescribe an order for headers, in
    our experience, many web-servers will fail if the host header does
    not appear first in request headers. We ensure this by manually
    adding the header ourselves rather than relying on hyper to do so.
• Use openssl instead of rusttls for outbound webhooks. In our experience,
  many widely deployed webservers use "weak" ciphers that rusttls refuses to
  support, leaving us with no choice but to rely on openssl libraries.
• Support for soft-limiting of concurrent worker tasks. Unbounded concurrent worker
  tasks can easily overwhelm a system. This sets a default of 500 tasks with support
  for configuring other limits or setting unlimited (0) concurrent tasks
• Refactor message dispatch code for clarity. Dispatch code is now broken into
  separate methods: prepare_dispatch, make_http_call,
  handle_successful_dispatch, and handle_failed_dispatch with the dispatch itself
  having different types depending on its stage in the dispatch process.
  Other function names have also been changed for improved clarity.
• Prevent worker from shutting down when there are active tasks.
• Avoid multiple DB calls for message-destination insert/retrieval