timeshift: add unlimited size/period options.
remove debug left in my mistake.
Ensure timeshift unlimited period flag is obeyed.
Added on-demand buffer mode.

In this mode data will only be written to disk when paused and anything
in the buffer prior to the current play point will immediately be removed.

This implies that rewinding is never possible in this mode (though FF is)
and trying to do so will result in the buffer playback being paused.
Fix #1471 - add explicit type cast.
timeshift: Reworked the reader engine to common up things and prepare for seek support.
timeshift: remove unused/incomplete code.