Improve mapper performance

[filipsobol]

Suggested merge commit message (convention)

Other (engine): Improve performance of the getModelLength method.

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While this looks like a simple change, it introduces significant performance improvements by avoiding recursion and calling this._viewToModelLengthCallbacks.get() on nodes without name or when the map is empty (most cases).

Test name	Improvement in %	Improvement in ms
lists	-4.9%	-100ms
formatting-long-paragraphs	-15.0%	-730ms
inline-styles	-22.2%	-2310ms
wiki	-4.0%	-140ms

We could gain even more performance by removing the public registerViewToModelLength method and the this._viewToModelLengthCallbacks map, since it's only used in legacy lists, which we are deprecating. However, it's likely that it's used by the integrators and may not be possible. This would improve the performance (on top of proposed changes) of formatting-long-paragraphs by another 180ms (-4.4%) and inline-styles by 300ms (-3.8%).

[scofalik]

Heads up - I was going to rewrite this piece of code in a different way with my Mapper improvements. @niegowski -- do not spent time reviewing it as it may change.