Restore leading event for move throttling
Restore leading event for move throttling

Hash updates turn out to be very timing sensitive.
While it's OK for drag gestures to be throttled with
a trailing edge, manual moves such as zooming to a
particular feature should always trigger a leading
edge update.

To recoup the performance, we could refactor the map
events to the standard movestart, move, and moveend,
and update the hash only on moveend.

Fixes #2023
kartta-labs/iD,mapmeld/iD,energiekollektiv/openmod.sh-id,energiekollektiv/openmod.sh-id,digidem/iD,openstreetmap/iD,1ec5/iD,tbicr/iD,zbycz/iD,ngageoint/hootenanny-ui,tbicr/iD,edpop/iD,kepta/iD,1ec5/iD,ngageoint/hootenanny-ui,openstreetmap/iD,mapmeld/iD,bagage/iD,morray/iD,AndreasHae/iD,ngageoint/hootenanny-ui,mapmeld/iD,digidem/iD,bagage/iD,kepta/iD,andreic-telenav/iD,edpop/iD,kartta-labs/iD,1ec5/iD,morray/iD,kartta-labs/iD,openstreetmap/iD,AndreasHae/iD,AndreasHae/iD,bagage/iD,edpop/iD,andreic-telenav/iD,zbycz/iD