fix :too many client creation

[ujisi]

If redis is not available, then Factory.create will be rejected, and generic-pool try to re-create the client. As a result, too many clients are created in heap. When redis becomes available, these clients can take all the free connections.

Steps for reproduce:

1. stop redis server
2. create pool
3. subscribe on event 'error'
4. call getConnection()

Sample code:

const poolOptions = {
    host: "127.0.0.1",
    port: "6379",
};
const redisPool = new RedisPool(poolOptions);

redisPool.on('error', (e, client) => {
  console.log(`error: ${e.message}`);
});

const client = redisPool.getConnection()
    .then(() => console.log('client create'))
    .catch(e => console.error(`client error: ${e.message}`));