Support the use of clamdscan for virus checking during ingest

The current virus checking during ingest is problematic for two reasons.

1) using the curation task to check for viruses means that all bitstreams are virus checked each time a bitstream is uploaded, see ClamScan.perform
2) using TCP to communicate with the clamav daemon is painfully slow for large bitstreams

This change allows the ingest to be configured to invoke clamdscan locally as a process.  This change does not address using clamdscan for curation tasks.