Support new readConcern level "linearizable"

A linearizable read avoids returning reads from a "stale" primary (one that has already been superseded by a new primary but doesn't know it yet). It is important to note that readConcern level linearizable does not by itself produce linearizable reads; they must be issued in conjunction with w:majority writes to the same document(s) in order to be linearizable.

This feature is exposed as a new value for readConcern.level, "linearizable".