Avoid static initialization of ObjectMappers

Various classes statically initialize an ObjectMapper READER instance. This ends up doing a bunch of class-loading of Jackson libraries that can add up to a fair amount of CPU, even if the reader ends up not being used. This is particularly the case with WebHdfsFileSystem, which is class-loaded by a serviceloader even when unused in a particular job. We should lazy-init these members instead of doing so as a static class member.