make archive table not required for MT startup

[replay]

With this change, on startup MT will still create the mt_index_archive table if the create-keyspace flag is true. But if it is false it won't error on startup, because actually that table is not required for it to run, it's only required for the mt-index-prune utility.

Fixes: #1234

[replay]

Tested in my QA instance:

• Ran the mt-index-prune utility with -create-keyspace=false and it errored with the expected message
• Started a read MT while the archive table did not exist, it came up correctly. The read MTs have -create-keyspace turned set to false
• Ran the mt-index-prune utility without specifying -create-keyspace (default is true) and the table got created correctly

[woodsaj]

lgtm other then minor typo in a comment.