Introduce immutable-table table property and if set, disallow insert-into

This patch introduces a new table property "immutable". If we create a table with TBLPROPERTIES("immutable"="true"), then INSERT INTO behaviour into that table will be disallowed if there is any data already present that the INSERT INTO would append to. INSERT INTO will still work if it is empty. INSERT OVERWRITE behaviour is not modified by this property, since an INSERT OVERWRITE's intent is effectively to drop and re-create. The desirability of having an immutable flag for a table is that it allows a table to be flagged to be protected against accidental updates due to a script loading data into it being run multiple times by mistake. With the flag set, the first insert would succeed, and successive inserts would fail, thus resulting in only one set of data in the table, instead of silently succeeding with 4 copies(say) of the data in the table.