Fixed pruning logic

- Erase an element from m_blocksByHeight only if it's empty
- Remove blocks from unconnected alternative chains too when they get old enough (4xPPLNS window)

src/pool_block.h

@@ -131,6 +131,8 @@ struct PoolBlock
 	bool m_broadcasted;
 	bool m_wantBroadcast;
 
+	time_t m_localTimestamp;
+
 	void serialize_mainchain_data(uint32_t nonce, uint32_t extra_nonce, const hash& sidechain_hash);
 	void serialize_sidechain_data();