Don't close the connection during range requests.

[yurydelendik]

Instead of auto-fetch mode we need to use already opened (initial) connection, in addition to range requests. That might speed up the loading PDF from the sites where range requests are slow. Firefox supports "moz-chunked-arraybuffer" responseType, so we start using data before it's completely loaded. There is also http://www.w3.org/TR/streams-api/#XMLHttpRequest

Pros:

• Less overhead on range requests, hence faster loading;
• Early access for the linearized data even if range requests are not available.

Cons:

• Two connections are opened: range request and initial one;
• We will fetch the same data chunks twice e.g. PDF tail (first time with range, second when entire data is received).

[yurydelendik]

Might help with #4791

[yurydelendik]

Fixed by #5263

[yurydelendik]

Follow up PR at #5319