[rbp/omxplayer] Handle displaytime more like dvdplayer

We had various glitches when mixing seeking and trickplay with online streams or dvd content.
This tries to match more closely what dvdplayer does and fixes some seek/trickplay jumping issues
[rbp/omxplayer] Avoid loss of volume messages

There seems to be a race condition where the volume request arrives as the omx audio pipeline is being created.
Restructure the volume messages to apply the initial volume as pipeline is being created, and add missing locks.
This also reduces the differences between OMXPlayer and DVDPlayer