sql: fix TestSchemaChangeReverseMutations
sql: fix TestSchemaChangeReverseMutations

The schema changes in the test were being processed correctly,
but every once in a while a SELECT following the schema changes
(running in a different thread via the asynchronous path) would
use an old lease on the table descriptor. This is correct behavior,
but makes the test unpredictable. The test now waits until the older
lease has been flushed before moving ahead with using the latest
lease on the table descriptor.

glycerine/cockroach,cockroachdb/cockroach,maxlang/cockroach,arjun4084346/cockroach,paperstreet/cockroach,veteranlu/cockroach,glycerine/cockroach,glycerine/cockroach,sunya123/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,veteranlu/cockroach,cockroachdb/cockroach,sunya123/cockroach,vivekmenezes/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,glycerine/cockroach,arjun4084346/cockroach,veteranlu/cockroach,maxlang/cockroach,vivekmenezes/cockroach,maxlang/cockroach,veteranlu/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,vaibhavarora/cockroach,cockroachdb/cockroach,glycerine/cockroach,veteranlu/cockroach,petermattis/cockroach,sunya123/cockroach,sunya123/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,vivekmenezes/cockroach,arjun4084346/cockroach,vaibhavarora/cockroach,vivekmenezes/cockroach,sunya123/cockroach,maxlang/cockroach,maxlang/cockroach,petermattis/cockroach,sunya123/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,glycerine/cockroach,petermattis/cockroach,vivekmenezes/cockroach,cockroachdb/cockroach,sunya123/cockroach,petermattis/cockroach,vivekmenezes/cockroach,arjun4084346/cockroach,veteranlu/cockroach,arjun4084346/cockroach,maxlang/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,maxlang/cockroach,petermattis/cockroach,maxlang/cockroach,paperstreet/cockroach,arjun4084346/cockroach,cockroachdb/cockroach,petermattis/cockroach,petermattis/cockroach,glycerine/cockroach,vivekmenezes/cockroach,petermattis/cockroach,veteranlu/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,arjun4084346/cockroach,paperstreet/cockroach,paperstreet/cockroach,cockroachdb/cockroach,sunya123/cockroach,glycerine/cockroach,arjun4084346/cockroach