Region reopen procedure should support some sort of throttling

Adds new configurations to control the speed and batching of region reopens after modifying a table: - hbase.reopen.table.regions.progressive.batch.size.max - When set, the HMaster will progressively reopen regions, starting with one region and then doubling until it reaches the specified max. After reaching the max, it will continue reopening at that batch size until all regions are reopened. - hbase.reopen.table.regions.progressive.batch.backoff.ms - When set, the HMaster will back off for this amount of time between each batch.