Annotator issues

[yifanmai]

• RunSpec.annotators should be named RunSpec.annotator_specs
• Annotators should support concurrency. AnnotatorFactory.get_annotator() should not enforce singletons. The image2structure annotators should be using locks instead. This is a correctness issue - currently, if multiple annotators with different arguments are instantiated, the run could get an annotator with the wrong arguments.
• Annotator cache directory should be based on the name field, rather than the class name. For instance, the folder for LatexCompilerAnnotator should be latex_compiler, not latexcompiler.
• Annotator frontend display should support most "simple" JSON objects, but currently only supports lists of dicts (and single dicts after Allow dicts to be displayed in AnnotationsDisplay without a enclosing list #2700)