This could lead to a noticeable (single-percent) performance
improvement.
Most of the functionality for this is in the packet analyzers that now
cache ignore_chesksums_nets.
Based on a patch by Arne Welzel (Corelight).
This also also combines the old TransportLayerAnalyzer class into
SessionAdapter, and removes the old class. This requires naming changes
in a few places but no functionality changes.