Local async for plugins, simplification, and performance

crew 0.6.0

• Migrate checks to enforce features in version 0.5.0 for reverse dependencies.
• Drop check of backlogged workers, c.f. shikokuchuo/mirai#63 (comment) (#79, #124, @shikokuchuo).
• Deprecate seconds_exit because exitlinger in mirai is now obsolete (#125, @shikokuchuo).
• Use mirai::nextget("cv") to count unresolved tasks instead of looping through all the task objects (#131).
• Remove throttling and collect() in auto-scaling. Simplifies much of the code. Made possible by the efficiency gains in #131.
• Simplify wait().
• seconds_interval in map() no longer defaults to controller$client$seconds_interval.
• launcher$terminate_workers() terminates one or more workers, and launcher$terminate terminates the whole launcher.
• Add infrastructure to let custom launcher plugins launch and terminate workers asynchronously (#133). Launchers can set a positive number in the processes field to set the number of local mirai daemons for asynchronous requests to launch and terminate the serious workers. Then, launch_worker() and terminate_worker() can optionally make use of launcher$async$eval() to send these asynchronous calls.