SERVER-12712 Remove dependency on auth_helpers from C++ client

We were only pulling in auth_helpers for the password digest. Instead
of doing that, move the digest function into the md5 library.
SERVER-12717 Remove unnecessary dependency on db/commands.h from fail points
SERVER-12718 The C++ driver doesn't actualy depend on task.cpp, so remove it
SERVER-12719 Remove unused trace.cpp from C++ driver, and rwlockimpl.cpp which supported it
SERVER-12720 Remove dependency on file and process info files from client driver
SERVER-12729 Remove unused histogram.cpp file from C++ client driver
SERVER-12736 askPassword is not used by the client driver, remove its source and header
SERVER-12737 Clean up includes of jsobjmanipulator.h