Rewrite rap-worker script in an optimized fashion

It now does much less directory reading, and performance should
scale far better with large numbers of recordings.

Semantics should be mostly unchanged, but there's greater use
of '.fail' files to mark errors, and '.done' files are now removed
after all of the following processing steps complete.

The rap worker no longer relies on processing scripts leaving
behind empty directories; those are now removed where appropriate.
bbb-record: Update 'rebuild' command for new rap-worker

The new rap worker relies only on .done files for triggering, rather
than the absence of process/publish directories.
record core: Remove hardcoded ffmpeg path

The ffmpeg binary should be looked up using the normal PATH mechanism,
it might not always be installed in the same place.
bbb-record: Improve steps done in --delete and --deleteall

It's now done in an order that should be less likely to cause
issues with a concurrent rap-worker process, and the files
deleted by --delete and --delete-all are now the same.
rap-worker: Set log level to 'INFO'

I had it on DEBUG temporarily for testing. The old version used ERROR,
but this made it print virtually no output which made diagnosing
issues difficult.