ROS 2 context shutdown doesn't play nice with email context shutdown

[christophebedard]

• The polling manager thread isn't properly shut down and joined.
  • Fix polling manager thread shut down #248
• There seems to be an error in the shut down order/context-related objects lifetime that produces a segfault.
  • Fix segfault in context shut down by fixing init/fini logic #251
• There's sometimes segfault on shutdown inside email::EmailReceiver::execute/email::CurlContext::execute/curl_easy_perform. It might be still trying to execute a request when we're trying to shut down.