Provide initial size for ColumnFamily object.

It's fix for https://issues.apache.org/jira/browse/CASSANDRA-3741.
The problem there is that when ColumnFamily got all columns deleted, the size() method returns 0 despite the fact that actual object size is at least 40 bytes. 
It affects calculations of whole Memtable size. In my case I deleted millions of columns and Cassandra didn't calculate Memtable size correctly, and didn't flush it. It caused OOM crash of DB.