Calculating segment distances must not block user

[wingsofovnia]

It should be done async in background
Relates to #148

[wingsofovnia]

We decided to move responsibility to keep distances between stations to the generator itself since it's the only component that requires them. Therefore, distance generation is not a case for creating segments.

• Remove distance from Segments
• Create DistanceMap that will contain length of segments for schegen
• Implement methods for distance generation in DistanceMap
• Attache schegen to new DistanceMap rather direct access of distances from Segmenets
• Move distance related classes from transit to schedule module

[wingsofovnia]

Closes #125