Fix switchover when many connections appear

[noname0443]

Pull request description

This PR aims to fix the switchover when a lot of connections try to change data. MySync can't kill them fast enough and fails to complete the switchover.

Please provide steps to reproduce (if it's a bug)

1. Run the script:

docker rm -f $(docker ps -aq)

cd ../../
GOOS=linux go build -tags netgo,osusergo -o ./tests/images/mysql/mysync ./cmd/mysync/...
docker build --tag=mysync-test-base8.0 tests/images/base --build-arg MYSQL_VERSION=8.0
cd tests/images/
VERSION=8.0 docker-compose up --build -d

docker rm -f images-mysql3-1

docker cp images_zoo1_1:/etc/zk-ssl/ca.cert.pem .
docker cp images_zoo1_1:/etc/zk-ssl/server.crt .
docker cp images_zoo1_1:/etc/zk-ssl/server.key .

docker exec -it images_mysql1_1 mkdir -p /etc/zk-ssl
docker exec -it images_mysql2_1 mkdir -p /etc/zk-ssl

docker cp ca.cert.pem images_mysql1_1:/etc/zk-ssl/
docker cp ca.cert.pem images_mysql2_1:/etc/zk-ssl/

docker cp server.crt images_mysql1_1:/etc/zk-ssl/
docker cp server.crt images_mysql2_1:/etc/zk-ssl/

docker cp server.key images_mysql1_1:/etc/zk-ssl/
docker cp server.key images_mysql2_1:/etc/zk-ssl/

echo "Trying to add mysql1 to mysync..."
while
  timeout -k 70 60 docker exec images_mysql1_1 mysync host add mysql1
  [ $? != 0 ]
do
  echo "Let's try for another time..."
  sleep 2
done

echo "Trying to add mysql2 to mysync..."
while
  timeout -k 70 60 docker exec images_mysql1_1 mysync host add mysql2
  [ $? != 0 ]
do
  echo "Let's try for another time..."
  sleep 2
done

docker exec images_mysql2_1 mysync host add mysql1
docker exec images_mysql2_1 mysync host add mysql2

docker exec images_mysql1_1 mysql -e "CREATE TABLE test1.big_table (id integer, info text);"

docker exec images_mysql1_1 mysql -e "INSERT INTO test1.big_table (id, info) VALUES (1, 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa');"
for ((a = 0; a < 8; a++)); do
  docker exec images_mysql1_1 mysql -e "INSERT INTO test1.big_table SELECT * FROM test1.big_table;"
done

docker exec images_mysql1_1 mysql -e "CREATE USER user1 IDENTIFIED BY 'password';"
docker exec images_mysql1_1 mysql -e "GRANT ALL ON test1.big_table TO user1;"

2. Use sysbench with a lot of threads or something like that:

while true; do
  docker exec images_mysql1_1 mysql -u user1 -ppassword -e "INSERT INTO test1.big_table (id, info) VALUES (SLEEP(1), 'aaaaaaaaaaaaa');" &
  sleep 0.01
done

3. Try to do switchover... It will fall.