fix downsampling interval overflow issue

When the downsampling interval is greater than max integer, it will overflow and the call will fail:
{"error":{"code":500,"message":"interval not > 0: -1702967296","trace":"java.lang.IllegalArgumentException: interval not > 0: -1702967296\n\tat net.opentsdb.core.TsdbQuery.downsample(TsdbQuery.java:253) ~[tsdb-2.0.0.jar:e39d053]\n\tat net.opentsdb.core.TSQuery.buildQueries(TSQuery.java:138) ~[tsdb-2.0.0.jar:e39d053]\n\tat net.opentsdb.tsd.QueryRpc.execute(QueryRpc.java:93) ~[tsdb-2.0.0.jar:e39d053]\n\tat net.opentsdb.tsd.RpcHandler.handleHttpQuery(RpcHandler.java:255) [tsdb-2.0.0.jar:e39d053]\n\tat net.opentsdb.tsd.RpcHandler.messageReceived(RpcHandler.java:163) [tsdb-2.0.0.jar:e39d053]\n\tat org.jboss.netty.channel.SimpleChannelUpstreamHandler.handleUpstream(SimpleChannelUpstreamHandler.java:70) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.channel.DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline.java:560) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.channel.DefaultChannelPipeline$DefaultChannelHandlerContext.sendUpstream(DefaultChannelPipeline.java:787) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.channel.Channels.fireMessageReceived(Channels.java:296) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.handler.codec.frame.FrameDecoder.unfoldAndFireMessageReceived(FrameDecoder.java:459) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.handler.codec.replay.ReplayingDecoder.callDecode(ReplayingDecoder.java:536) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.handler.codec.replay.ReplayingDecoder.messageReceived(ReplayingDecoder.java:435) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.channel.SimpleChannelUpstreamHandler.handleUpstream(SimpleChannelUpstreamHandler.java:70) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.channel.DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline.java:560) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.channel.DefaultChannelPipeline$DefaultChannelHandlerContext.sendUpstream(DefaultChannelPipeline.java:787) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.channel.SimpleChannelHandler.messageReceived(SimpleChannelHandler.java:142) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.channel.SimpleChannelHandler.handleUpstream(SimpleChannelHandler.java:88) [netty-3.6.2.Final.jar:na]\n\tat net.opentsdb.tsd.ConnectionManager.handleUpstream(ConnectionManager.java:87) [tsdb-2.0.0.jar:e39d053]\n\tat org.jboss.netty.channel.DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline.java:560) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.channel.DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline.java:555) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.channel.Channels.fireMessageReceived(Channels.java:268) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.channel.Channels.fireMessageReceived(Channels.java:255) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.channel.socket.nio.NioWorker.read(NioWorker.java:88) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.channel.socket.nio.AbstractNioWorker.process(AbstractNioWorker.java:107) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.channel.socket.nio.AbstractNioSelector.run(AbstractNioSelector.java:312) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.channel.socket.nio.AbstractNioWorker.run(AbstractNioWorker.java:88) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.channel.socket.nio.NioWorker.run(NioWorker.java:178) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.util.ThreadRenamingRunnable.run(ThreadRenamingRunnable.java:108) [netty-3.6.2.Final.jar:na]\n\tat org.jboss.netty.util.internal.DeadLockProofWorker$1.run(DeadLockProofWorker.java:42) [netty-3.6.2.Final.jar:na]\n\tat java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886) [na:1.6.0_35]\n\tat java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908) [na:1.6.0_35]\n\tat java.lang.Thread.run(Thread.java:662) [na:1.6.0_35]\n"}}