(SERVER-61) ExecutionStub: consume STDERR/STDOUT

Add a ProcessWrapper class for use by the ExecutionStub, which ensures
that the child process used to execute the command safely gets its STDOUT and
STDERR consumed by the parent process.  If this is not done, deadlock
can occur, if the child process is blocked on writing to either STDOUT
or STDERR, and the parent process is waiting on the child process to
terminate.