feat($concurrent-chunks): start refactor of upload controller to support concurrent chunk uploads/file #937

- Centralize connection handling logic
- Expose new concurrent property for chunking option
- Stub for concurrent chunk unit tests
feat($concurrent-chunks): better connection manager method names #937
feat($concurrent-chunks): abc #937
chore(.jshintrc): `something == null` is ok
feat($concurrent-chunks): make concurrent chunk uploads possible for all endpoint types #937

The following features do not yet work with concurrent chunk uploads, and must be fixed:
- Progress tracking
- Canceling
- Retry or any failure handling whatsoever
- Auto-resume
- Scaling (blob proxies will not be eligible for concurrent chunking)
feat($concurrent-chunks): Fix progress reporting #937
feat($concurrent-chunks): Fix progress & auto-resume #937

Also some other code tidying.
feat($concurrent-chunks): Fix pause & cancel #937
feat($concurrent-chunks): Fix type error on chunk done #937
feat($concurrent-chunks): Auto-retry logic adjustment #937
chore(upload.handler.controller.js): log statements #937
test($concurrent-chunks): start of unit tests #937
fix($concurrent-chunks): fix cancel logic #937
fix($concurrent-chunks): fix retry logic #937
fix($concurrent-chunks): more retry logic adjustments #937
POST after last chunk success for traditional endpoints #937

Also:
- Added a proper Accept header for all requests aimed at integrator-controlled endpoints
 - We now account for reset responses on the "all chunks done" POST
test(unit/concurrent-chunks.js): test all chunks done POST #937
feat(upload-handler/xhr.upload.handler.js): version stored resume data  #937
test($accept): Update servers to look for Accept header
test($accept): Update servers to look for Accept header