Provide opt-in way to disable snapshotting/resetting for DbContext pooling

[roji]

Instead of using the built-in context pooling, I experimented with a non-resetting context pool which simply hands out instances without any state resetting etc.; this yielded a 1.8% perf increase. We can add an opt-in flag to DbContext pooling that would likely get use close to that.

Note similarity with Npgsql's No Reset On Close, which does the same at the ADO connection pooling level.

[roji]

We could add this as a flag, but it would be better to move the snapshotting/resetting logic to a place which the user can override, and simply not call them. #24768 and #24797 could go in this direction.