Change default of rocksdb_large_prefix in 8.0 to TRUE like InnoDB

Leave behind code that allows max key part length to be 3072 bytes.  From discussion with PZ from a user "In MySQL 8 the default character set is utf8mb4 meaning single VARCHAR(255) column is more than 1000 bytes and as such can't be indexed with default settings in PS 8 myrocks."

Change the default value in 8.0 to TRUE like InnoDB.  During the 8.0 port, many tests were altered to use shorter keys to stay within the smaller limit.  Lets revert these specific changes to capture/test the new default for rocksdb_large_prefix.