Introduce MetaEditor method that both adds and deletes rows in .META. table

In review of HBASE-7365, MetaEditor.deleteRegions() and MetaEditor.addRegionsToMeta() are used in RestoreSnapshotHandler.java.handleTableOperation() to apply changes to .META.

I made following suggestion:

Can we introduce new method in MetaEditor which takes List of Mutation's ?
The Delete and Put would be grouped and then written to .META. table in one transaction.

Jon responded:

I like that idea – then the todo/warning or follow on could refer to that method. When we fix it, it could get used in other multi row meta modifications like splits and table creation/deletion in general.

See https://reviews.apache.org/r/8674/