* Removes the initialClusterTime and initialOperationTime session
options.
* Enables causal consistency by default when using an explicit
session.
* Adds a --retryWrites command line option to the mongo shell for
enabling retryable writes in the mongo shell. The retryWrites
options to SessionOptions is left for convenience with testing.
* Renames setClusterTime() to advanceClusterTime(), and adds a
corresponding advanceOperationTime() method to DriverSession.
* Enables assigning transaction numbers for write commands where
ordered=false.
* Prevents the mongo shell from sending afterClusterTime or assigning
transaction numbers when talking to a stand-alone mongod.
* Prevents the mongo shell from assigning transaction numbers when
using an unacknowledged (w=0) writeConcern.
* Changes DBClientRS to re-discover the current primary of the replica
set when it receives an error code representing "not master" in
addition to an error message representing "not master".
* Adds a shellPrint() pretty-printer for SessionOptions and
DriverSession instances so they no longer print out their entire
object definition.
