Add option to prevent implicit creation of column family in MyRocks

Column Families in MyRocks maintain their own mem-table and are always open. Mem-tables consume a fair amount of memory outside of the block cache. Since the creation of a Column Family has a direct impact on the memory footprint of the mysqld process, users should not always have unrestricted, free-reign to implicitly create column families at will. Consider the following example:

CREATE TABLE t1(
  a INT,
  b INT,
  c INT,
  d INT,
  PRIMARY KEY(a) COMMENT 'the primary key',
  KEY ab(a,b) COMMENT 'key ab',
  KEY bc(b,c) COMMENT 'key bc',
  KEY cd(c,d) COMMENT 'key cd',
  KEY abcd(a,b,c,d) COMMENT 'key abcd') ENGINE=ROCKSDB;

This will result in MyRocks creating Column Families by the names of:

• 'the primary key'
• 'key ab'
• 'key bc'
• 'key cd'
• 'key abcd'

The fact that any index comment will result in a new column family is a different problem unto itself (see PS-4947), but, this now can not be changed in 5.7.  Regardless, even if the intention was to create a new column family, due to the impact on the required resources, it should be a managed/privileged option at the minimum controlled by a read only server variable.