Log time spent waiting for read concern

This is a request similar to SERVER-18067.

At present the only way to infer if a query might have waited for read concern is by checking for the lag at the time of query execution. This is not straightforward and can still be inconclusive, especially if the query execution time & lag are under 1 second (i.e. t2 resolution is limited to 1s).

Would be nice to have the time spent by a query waiting for read concern as a separate item in the log message.