Convert systemd notification to prelaunch steps

Introduce the rabbit_boot_state module, which extracts boot state
management out of rabbit_prelaunch.

External boot state listeners, such as systemd, now live under the
rabbit_boot_state_sup supervisor, which dispatches boot state updates to
all of its children as a gen_server cast.

Additionally:
- the systemd listener now reads the NOTIFY_SOCKET env var directly,
  rather than through rabbit_env, to avoid the need to wait for the
  rabbit_env context to initialize
- the sytemd listener now only checks for the sd_notify module and
  NOTIFY_SOCKET env var once upon startup, exiting gracefully when not
  needed
- systemd related log messages are now routed through lager

0 files changed, 0 insertions, 0 deletions