Continue file upload

[jrchudy]

The changes made in this PR for ermrestJS better communicate the status of the current upload job after each chunk is successfully uploaded and resuming an upload job if there is a pending one tracked locally that is verified by the server.

The changes include:

• adding a chunkTracker for tracking which chunks in chunks[] are marked as complete determined by chaise which manages the local upload state
• store the startChunkIdx for initializing chunkTracker and initializing the upload progress on resume
• upload.prototype.fileExists accepts a jobUrl parameter that it will use as the chunkUrl if a 409 is returned when checking if the file exists (more details in the comments in the code)
• upload.prototype.start accepts a startChunkIdx to initialize chunkTracker and for skipping which chunks are added to the chunkQueue to avoid re-uploading chunks that already exist for this upload job
• Chunk.prototype.sendToHatrac will update chunkTracker when each chunk has finished uploading

These changes resolve step 1 of issue #2379 in chaise