Unify CQLTester#waitForIndex and SAITester#waitForIndexQueryable

It has been mentioned in CASSANDRA-18217 that CQLTester#waitForIndex and SAITester#waitForIndexQueryable can be unified into a single method.

The same discussion mentions that it's easy to forget to call those methods after calling CQLTester#createIndex. So that method can be renamed to createIndexAsync, and we can create a new method that creates the index and waits for it (createIndexSync, createIndexAndWaitForQueryable, or something like that).