Fix issue of mining rejig happening iff chain changed AND none ready:

Two blocks are mined on the same parent in quick succession. First gets
imported alone, chain changes but there's another "ready" so mining
doesn't get rejigged. When the second gets imported soon after, it
doesn't change the chain and so mining still doesn't restart.

libethcore/Farm.h

@@ -68,6 +68,7 @@ public:
 	void setWork(WorkPackage const& _wp)
 	{
 		WriteGuard l(x_minerWork);
+		cdebug << "Farm::setWork()";
 		if (_wp.headerHash == m_work.headerHash)
 			return;
 		m_work = _wp;
@@ -94,6 +95,7 @@ public:
 	void stop()
 	{
 		WriteGuard l(x_minerWork);
+		cdebug << "Farm::stop()";
 		m_miners.clear();
 		m_work.reset();
 		m_isMining = false;
@@ -175,6 +177,7 @@ private:
 	bool start()
 	{
 		WriteGuard l(x_minerWork);
+		cdebug << "start()";
 		if (!m_miners.empty() && !!std::dynamic_pointer_cast<MinerType>(m_miners[0]))
 			return true;
 		m_miners.clear();

libethereum/Client.cpp

@@ -702,7 +702,7 @@ void Client::onChainChanged(ImportRoute const& _ir)
 
 	// RESTART MINING
 
-	if (!m_bq.items().first)
+	if (!isSyncing())
 	{
 		bool preChanged = false;
 		State newPreMine;