StreamsFragment: Check for the listplaylist command availablility before run.

This is like a96a211, but for StreamsFragment as it often asks for it in
FSFragment.

Like a96a211, this only works on the standard MPD implementation for 0.19.2 and
greater.
MPDConnection: Throw IOException if no response and no error.

We've received a couple of automated crash reports which no exception was thrown
and no result was received. This is probably a bug of some sort. Having the user
crash isn't going to help, throw an IOException for now, until the bug is either
found or the code is replaced.
use MPD's own database update time when checking for album cache

should fix cache update problems if the server's and the android client's
times are not synchronized, see #645
make backup of value before setting it