Conversion of opentxs-client member functions to const

This is the first phase of Ruben's const-correctness conversion of opentxs-client
Next phase will be the function parameters.
Looking at the names of the functions I cannot understand why these can be const. Discard without parameters even. Doesn't action mean change in state?!

This is not the only place where this applies - it in for many of the const functions.