Bring ALTER ... DROP COMPACT STORAGE out of experimental

ALTER ... DROP COMPACT STORAGE statements have not been extensively tested and suffer from several issues like:

• As COMPACT tables did not have primary key liveness there empty rows
  inserted AFTER the ALTER will be returned whereas the one inserted before
  the ALTER will not.
• Also due to the lack of primary key liveness the amount of SSTables being
  read will increase resulting in slower queries (CASSANDRA-16675)
• After DROP COMPACT it becomes possible to ALTER the table in a way that
  makes all the row disappears
• There is a loss of functionality around null clustering when dropping
  compact storage (CASSANDRA-16069)

To avoid running into those issues, CASSANDRA-16733 introduced a new flag that allows operators to disable those statements on their clusters and pointed out that ALTER ... DROP COMPACT STORAGE is not production-ready.

see https://www.mail-archive.com/dev@cassandra.apache.org/msg16789.html

This ticket should facilitate outlining and implementing a plan how to make ALTER ... DROP COMPACT STORAGE production-ready and revert the patch from CASSANDRA-16217.