Get current segment number from segment filename when updating HLS storage sequences

[lastcanal]

The current implementation is not smart enough to discover when discontinuities have caused the segment number to increment, causing a mis-match between the stored sequence numbers and the current published 'preload-hint' partial.

This PR uses the filename to get the segment number but could be improved by including the segment number in the metadata sent by Membrane.HTTPAdaptiveStream.Storage

fixes #133