[rpi] Fix bcm2835_audio locking and samples starvation

Rework general approach to locking and working with audio worker thread:

• Use flags to signal requested worker action
• Fix submitted buffer calculations to avoid samples starvation
• Protect buffer pointers with locks to fix race condition between callback and audio worker thread
• Remove unnecessary vchi_service_use
• Do not use lock to serialize VCHI requests since only one thread issues them now
• Fix unloading signaling per hselasky@ suggestion
• Add output to detect inconsistent callback data caused by possible firmware bug https://github.com/raspberrypi/firmware/issues/696
• Add stats/debug sysctls to troubleshoot possible bugs

PR: 213687, 205979, 215194
MFC after: 1 week