be more data plan friendly by setting DirCache 0 when possible

The default of "DirCache 1" causes all (two) consensus versions to be downloaded when the typical client use on a mobile phone only requires one version. Either set DirCache automatically depending on the given configuration options or add a GUI option with a default of 0. Note that most users of orbot will probably not thoroughly read the orbot man page, have a look at the source code and then use the custom torrc option, thus superfluous data will be downloaded which is not so nice with a mobile data plan.

Trac:
Username: anstein