Userstats stop populating using multiple replication threads

• Bringing back number of threads to 0 is a workaround

To reproduce:
Setup master slave replication

Produce some writes on the master

On the slave:
SET GLOBAL userstat=1;
SHOW TABLE_STATISTICS;

Statistics is populated

STOP SLAVE;
FLUSH TABLE_STATISTICS;
SET GLOBAL slave-parallel-threads=16;
START SLAVE;
SHOW TABLE_STATISTICS;

Statistics is not populated