Remove the write-blocking mode of dbCheck

The point-in-time snapshot mode of dbCheck (the default) seems to have been working well. Given that the default history window is 5 minutes, there is less of a risk of secondaries not having enough history to support a timestamped read in the past. This was more of a concern in 4.4 where the window was 5 seconds.

We should remove support for the version that uses MODE_S locks and blocks writes, since it does not appear to be in active use.