Add tracing to MCAS allocator and GC

Add tracing to MCAS allocator and GC

Adds conditional trace logging to MCAS object cache and supporting GC,
using ViceLog.  While in future this should be normalized, it is correct
for both fileserver and volserver, the two programs using MCAS currently.

LICENSE BSD

Reviewed-on: http://gerrit.openafs.org/374
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>