Debug assertion in sts_combinations in readwritesplit

The sts_combinations test sometimes fails with the following debug assertion:

1728021222.781: error  : (2237) [readwritesplit] (<1> service51); debug assert at /home/vagrant/MaxScale/server/modules/routing/readwritesplit/rwsplitsession.cc:962 failed: m_current_query should not be empty if retry_failed_reads is enabled. (!m_config->retry_failed_reads)\n

This is caused by the fact that the mxs::Reply passed to handleError is not a reliable source of information for detecting whether a reply had started or not. This can be solved by tracking it inside readwritesplit as the return value of mxs::Reply::has_started() is reliable inside the clientReply function.