2i key cache load fails

CASSANDRA-9265 changed how key cache content is serialized to disk. It uses UUID cfId to generate the file path for each ColumnFamilyStore.

Since cfId of a secondary index is the same as for the base table, the key-cache files for 2i's and the base are the same. This will/may lead to deserialization failures on restart for tables with at least one 2i.

/cc aweisberg danchia