Body parser max length fix up

Fixes #2219 and #1666

Made many changes, including:

* Java body parsers let the Scala body parser enforce the max length.
* A new configuration item, parsers.disk.maxLength, created for
  specifying the default max length for parsers that buffer content to
  disk (documented in body parser docs and migration guide)
* Max length changed to Long for disk types, so that bodies greater than
  2GB can be parsed.
* Max data length for `multipart/form-data` bodies is now enforced
  across the aggregate of all data parts.
* Deprecated the RequestBody.isMaxBodySizeExceeded flag, since 413 is
  now returned directly from all body parsers if the max length is
  exceeded.
Merge pull request #2219 from jroper/2219-maxlength

RequestBody.isMaxSizeExceeded returns incorrect result for request body encoded in multipart/form-data