HTTP2 / pipelining support

[Komzpa]

Using HTTP Pipelining or HTTP2/SPDY, you can increase speed of downloading tiles on high latencies: first send a set of requests for tiles, then parse the responses for all them, not having a queue with latencies.

Difference in loadtimes can be seen opening https://osm.org (browsing in non-cached region on your tile proxy, ie US in Belarus) in Chrome and in QGIS/QMS.

[simgislab]

this should be part of TMS driver for GDAL, which QGIS/QMS will use

current QMS' implementation of TMS driver should probably be abandoned