Fix: never reset m_cumulativeHashes

Hashrates in stratum status can be wrong if `m_cumulativeHashes` decreases.

src/stratum_server.cpp

@@ -529,8 +529,7 @@ void StratumServer::reset_share_counters()
 {
 	WriteLock lock(m_hashrateDataLock);
 
-	m_cumulativeHashes = 0;
-	m_cumulativeHashesAtLastShare = 0;
+	m_cumulativeHashesAtLastShare = m_cumulativeHashes;
 	m_cumulativeFoundSharesDiff = 0.0;
 	m_totalFoundShares = 0;
 	m_totalFailedShares = 0;