Event-driven wait()

crew 0.7.0

• Migrate from asyncdial to autoexit.
• Use Sys.info()[["user"]] to get the user in crew_clean() (#135, @luwidmer).
• Use condition variables to wait for tasks more efficiently (#108).
• Because of #108, controller$map() can no longer be used if there are tasks waiting to be popped.
• Use a cli progress bar in map().
• Encapsulate non-function R6 members in the private list and add active bindings where interfaces are necessary (#137). Ad hoc tests that absolutely need to modify private objects can use object$.__enclos_env__$private trick, but these tests should be skipped on CRAN in case there is a change to R6 that breaks this.
• Drop .signal from mirai::mirai() since all mirai tasks signal as of mirai version 0.11.2.9025.
• Implement crew_throttle(), a decoupled mechanism for throttling that can be applied to scaling.
• Bring back the throttle argument, powered by crew_throttle().
• Retry mirai::status() again in daemons_info() and make it configurable using seconds_interval and seconds_timeout in both the client and the launcher (#128).