getClient(connection) {
        for (let i = this._pool.length - 1; i >= 0; i--) {
            if (this.sameConnection(connection, this._pool[i][0])) {
                return this._pool[i][1];
            }
        }

        const client = this.newClient(connection);
        this._pool.push([connection, client]);
        return client;
    }
Get client indexed by connection (if no client found, a new client is created using newClient)
@param {object} connection  Connection object
@returns {*} client for connection