9.0.0-rc1

See last link for the details about the following highlights:

• Link to new API docs - http://www.drush.org/api/master/index.html.
• The debug logs come out as notice (#3201)
• Add skip-tables, structure-tables and tables to @optionset_table_selection (#3195)
• Add a composer.json file to the output of the 'drush gen dcf' command. Enhance documentation on Drush commands. (#3191)
• Show shortcut in drush topic core:global-options
• updatedb: return exit code 1 on failure (#3188)
• Check to see if Drush extensions are compatible with the current version. (#3189)
• Simplified site aliases. Fixes #3107 (#3184)
• Set site alias search depth to '<=1' instead of '==0' to allow site aliases to be grouped together in directories.
• Fix drush si --db-su option
• Remove dupe backend result setter. (#3182)
• Use Symfony table instead of Console table (which is now removed)
• Fix #3168. Change colors for IO warning format
• Fix #3167. Don't preview or run entity updates by default. (#3177)
• Remove drush_directory_cache($subdir) in favor of Path::join($this->getConfig()->cache(), $subdir) (#3160)
• Add Prefllight logger. Use in RedispatchSiteLocal. Refs #3110 (#3154)
• Validate legacy command file (#3158)
• Changes since 9.0.0-beta9.