Allow a limit to be set on expiry of line segments

[tomhughes]

Accidentally or deliberately moving a node a long way creates a long line segment which can expire large numbers of tiles causing disruption to the rendering process as osm2pgsql either runs out of memory or generates a huge list of tiles to expire.

Limiting expiry of long line segments prevents that and also helps reduce disruption of the resulting rendered maps with long lines that are almost certainly incorrect.