Added exists() test to BlockRepository & HSQLDB implementation

src/main/java/org/qora/repository/BlockRepository.java

@@ -37,6 +37,11 @@ public interface BlockRepository {
 	 */
 	public BlockData fromHeight(int height) throws DataException;
 
+	/**
+	 * Returns whether block exists based on passed block signature.
+	 */
+	public boolean exists(byte[] signature) throws DataException;
+
 	/**
 	 * Return height of block in blockchain using block's signature.
 	 * 

src/main/java/org/qora/repository/hsqldb/HSQLDBBlockRepository.java

@@ -88,6 +88,15 @@ public class HSQLDBBlockRepository implements BlockRepository {
 		}
 	}
 
+	@Override
+	public boolean exists(byte[] signature) throws DataException {
+		try {
+			return this.repository.exists("Blocks", "signature = ?", signature);
+		} catch (SQLException e) {
+			throw new DataException("Unable to check for block in repository", e);
+		}
+	}
+
 	@Override
 	public int getHeightFromSignature(byte[] signature) throws DataException {
 		try (ResultSet resultSet = this.repository.checkedExecute("SELECT height FROM Blocks WHERE signature = ? LIMIT 1", signature)) {