Re-use the same EntityEntry in modifiers

[jtheisen]

Currently EntityEntry is requested from the context in each modifier gratuitously. This is slightly cumbersome to change as EF6 instead has a DbEntityEntry wich behaves the same, but the type difference (and the fact it's a generic type) means one probably has to wrap it to use it as a parameter in the modifiers.

This would be a somewhat sensible optimization.