optimize FileTools.toByteArray

Fixes the following autobug

"qtp1614100844-126" prio=10 tid=0x00007f500005b000 nid=0x49c3 runnable [0x00007f4fef8f4000]
   java.lang.Thread.State: RUNNABLE
        at com.newrelic.agent.Transaction.startFlyweightTracer(Transaction.java:1262)
        at org.eclipse.jetty.server.HttpInput.read(HttpInput.java)
        at org.eclipse.jetty.server.HttpInput.read(HttpInput.java:49)
        at com.geeksville.util.FileTools$$anonfun$toByteArray$1.apply(FileTools.scala:33)
        at com.geeksville.util.FileTools$$anonfun$toByteArray$1.apply(FileTools.scala:33)
        at scala.collection.immutable.Stream$.continually(Stream.scala:1129)
        at scala.collection.immutable.Stream$$anonfun$continually$1.apply(Stream.scala:1129)
        at scala.collection.immutable.Stream$$anonfun$continually$1.apply(Stream.scala:1129)
        at scala.collection.immutable.Stream$Cons.tail(Stream.scala:1085)
        - locked <0x00000007d4c32ec0> (a scala.collection.immutable.Stream$Cons)
        at scala.collection.immutable.Stream$Cons.tail(Stream.scala:1077)
        at scala.collection.immutable.Stream$$anonfun$takeWhile$1.apply(Stream.scala:803)
        at scala.collection.immutable.Stream$$anonfun$takeWhile$1.apply(Stream.scala:803)
        at scala.collection.immutable.Stream$Cons.tail(Stream.scala:1085)
        - locked <0x00000007d4c32ef0> (a scala.collection.immutable.Stream$Cons)
        at scala.collection.immutable.Stream$Cons.tail(Stream.scala:1077)
        at scala.collection.immutable.Stream$$anonfun$map$1.apply(Stream.scala:376)
        at scala.collection.immutable.Stream$$anonfun$map$1.apply(Stream.scala:376)
        at scala.collection.immutable.Stream$Cons.tail(Stream.scala:1085)
        - locked <0x00000007d4c32f40> (a scala.collection.immutable.Stream$Cons)
        at scala.collection.immutable.Stream$Cons.tail(Stream.scala:1077)
        at scala.collection.immutable.Stream.length(Stream.scala:284)
        at scala.collection.SeqLike$class.size(SeqLike.scala:106)
        at scala.collection.AbstractSeq.size(Seq.scala:40)
        at scala.collection.TraversableOnce$class.toArray(TraversableOnce.scala:248)
        at scala.collection.AbstractTraversable.toArray(Traversable.scala:105)
        at com.geeksville.util.FileTools$.toByteArray(FileTools.scala:33)
        at com.geeksville.dapi.MissionUploadSupport$class.handleMissionUpload(MissionUploadSupport.scala:32)