[meta] Implement full-featured drush commands for D8 migrate

Implement the most important D7 migrate drush commands for D8 migrate. The top priority is to implement the basic functionality of these commands:

• #2403409: Implement migrate-import command for D8
• #2403411: Implement migrate-rollback command for D8
• #2403417: Implement migrate-status command for D8
• #2403421: Implement migrate-stop command for D8

These are follow-up issues to fill in lower-priority bits of the above:

• #2429105: Display migration status (importing, rolling back) with drush migrate-status
• #2429107: Display last imported time with drush migrate-status
• #2429109: Support --refresh on drush migrate-status
• #2432977: Implement --idlist option on migrate-import
• #2432981: Implement --update option on migrate-import
• #2432983: Implement --limit option on migrate-import
• #2432985: Implement --feedback option on migrate-import
• #2432991: Implement --force option on migrate-import
• #2432993: Implement reporting of results on migrate-import

The following commands would be implemented in a second phase:

• #2630718: Implement drush migrate-fields-destination
• #2630720: Implement drush migrate-fields-source
• #2630722: Implement drush migrate-mappings
• #2561455: Implement drush migrate-messages command for D8 migrate
• #2561855: Implement drush migrate-reset-status command for D8 migrate

The following commands seem to be little-used - they will be implemented as the need appears, following the rest:

• migrate-analyze
• migrate-audit
• migrate-wipe (frankly, a command to delete all nodes of a given content type seems like it belongs in the devel module instead)