new command line flag: postpone-shutdown-flag-file, new server command; 'state'

[shlomi-noach]

Adding:

• --postpone-shutdown-flag-file: If indicated file exists, and assuming execution is successful, gh-ost will wait after cut-over, and after running the success hooks, instead of exiting. This can be useful in state based environments where we may wish to ask gh-ost wat the state of the migration is, as opposed to assuming it reports the state via hooks. This gives a controller/operator the chance to communicate to gh-ost and confirm that the migration is successful. See also the state server command.
• state server command: returns Running or Complete. For the latter, see --postpone-shutdown-flag-file