Added Range support to FileBackend::streamFile()

Added Range support to FileBackend::streamFile()

* Added HTTP options headers parameter to streamFile().
* Refactored doStreamFile() to either call StreamFile::stream()
  or delagate that to the subclass. SwiftFileBackend now relays
  the full Swift response rather than manually making the headers.
  This also makes Range headers easy to support.
* Made use of this in img_auth.php for performance on private wikis.
* Elimate stat call in streamFile() for Swift if "headers" is empty.
* Refactored StreamFile a bit to inject request headers instead
  of using the $_SERVER global. A header options parameter is used
  instead, which also supports Range.
* Removed now unused prepareForStream().
* Cleaned up streamFile() unit tests.

Change-Id: I2ccbcbca6caabb8cf65bd6b3084cede2e6ea628a