Walkable area builder improvements

[hannesj]

Summary

As discussed in OTP meeting last week, we decided to do performance improvements for the walkable area builder at a later stage. This pull request contains two of those. The first one is limiting the visibility nodes to only use convex points in a polygon. This reduces the build time of the Norwegian graph from ~3 minutes and 45 seconds to ~2min 15 seconds. The second one caches the way properties, which further reduces the build time to ~2minutes.

This also fixes a couple of issues with the builder:

• Do not split edges less than 10 cm.
• Make sure that all area edges are connected properly
• Run island pruning only after stop linking, even when building the street graph separately
• Add issue about unconnected areas

Unit tests

Unit test for calculating convex points in a polygon added

Code style

Code style followed

Documentation

No updates required

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[leonardehrenfried]

I checked it with one of my data sets and it looks good.