Preconnect to tiles server using current map / Preconnect к серверу с мозайкой карт

[kabalin]

There was hardcoded pre-connect to kosmosnimki which was removed in ee45efe (#422) since we don't use it by default any more and URL has changed in #378.

Might be useful to have this re-implemented as @aeifn suggested, i.e. preconnect using current user map before map is initialised in Leaflet. Can be addressed as part of #279 or seprately.

More info: https://www.igvita.com/2015/08/17/eliminating-roundtrips-with-preconnect/