It's impossible to re-enable the quota table if it gets disabled

HMaster.enableTable tries to read the quota table. If you disable the quota table, this fails. So then it's impossible to re-enable it. The only solution I can find is to delete the table at this point, so that it gets recreated at startup, but this results in losing any quotas you had defined.  We should fix enableTable to not check quotas if the table in question is hbase:quota.