Restructure relationships in street model

[leonardehrenfried]

Summary

This PR restructured the relationships between the following classes in the street model:

• Graph
• StreetIndex
• VertexLinker
• TemporaryVerticesContainer

Before this PR their responsibilities were very unclear with many cyclic relationships. I've tried to split up the responsibilities this way:

• Graph: top level mutable repository of street entities
• StreetIndex: package-private index of street entities for fast access, only accessible by Graph
• VertexLinker: stateless and only responsible for linking, nothing else
• TemporaryVerticesContainer: request-scoped helper object that orchestrates linking tasks and cleanup

Improvements

The Graph, index or VertexLinker don't require a site repository anymore.

Unit tests

Updated, but no new ones.

Documentation

JavaDoc

Bumping the serialization version id

Yes, because the graph has changed.

[codecov]

Codecov Report

Attention: Patch coverage is 83.18966% with 39 lines in your changes missing coverage. Please review.

Project coverage is 70.96%. Comparing base (a50b595) to head (b4520ad).
Report is 1 commits behind head on dev-2.x.

Files with missing lines	Patch %	Lines
...org/opentripplanner/routing/graph/StreetIndex.java	91.02%	5 Missing and 2 partials ⚠️
.../inspector/vector/vertex/VertexPropertyMapper.java	0.00%	5 Missing ⚠️
...nner/street/search/TemporaryVerticesContainer.java	91.22%	1 Missing and 4 partials ⚠️
...r/ext/debugrastertiles/EdgeVertexTileRenderer.java	0.00%	4 Missing ⚠️
...lanner/inspector/vector/edge/EdgeLayerBuilder.java	0.00%	3 Missing ⚠️
...vector/geofencing/GeofencingZonesLayerBuilder.java	0.00%	3 Missing ⚠️
...er/inspector/vector/vertex/VertexLayerBuilder.java	0.00%	3 Missing ⚠️
...pentripplanner/routing/graph/EdgeSpatialIndex.java	76.92%	1 Missing and 2 partials ⚠️
...aph_builder/module/OsmBoardingLocationsModule.java	85.71%	0 Missing and 1 partial ⚠️
...aph_builder/module/islandpruning/PruneIslands.java	66.66%	1 Missing ⚠️
... and 4 more

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #6605      +/-   ##
=============================================
- Coverage      70.98%   70.96%   -0.02%     
- Complexity     18272    18277       +5     
=============================================
  Files           2000     1999       -1     
  Lines          75871    75833      -38     
  Branches        7785     7778       -7     
=============================================
- Hits           53856    53818      -38     
+ Misses         19268    19267       -1     
- Partials        2747     2748       +1     

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