Race condition in MemB with concurrent non-intrusive backup can cause hang in index instance delete

The MemDB close() shuts down all the GC workers. GC workers are
responsible for delta backup writing. If a backup task is initiated
at the same time as MemDB.Close(), backup task would send state change
messages to GC workers. But, GC workers would have been shutdown by that
time. This would cause backup task to wait indefinitely for ack from GC
workers.