WL#1071: Add COPY TABLE functionality

Affects: Server-7.1   —   Status: Un-Assigned   —   Priority: Medium

Mysql backup already does this in a better way.

Additionally to  renaming tables we need to be able copy table efficiently.
At least for MyISAM tables we can just do it by copying files.

Copy of table can be convenient in many cases starting from experiments and
ending with shadow copy for long read only queries.