ACCUMULO-118 use a configurable class to select volumes for new files

git-svn-id: https://svn.apache.org/repos/asf/accumulo/branches/ACCUMULO-118@1494310 13f79535-47bb-0310-9956-ffa450edef68
ACCUMULO-118 cleanup TODOs simplify VolumeManager interface, add some basic documentation

git-svn-id: https://svn.apache.org/repos/asf/accumulo/branches/ACCUMULO-118@1494340 13f79535-47bb-0310-9956-ffa450edef68