Explore a whole directory hierarchy, looking for files to tail.
DirTailingAdaptor will not try to start tailing a file more than once,
if the file hasn't been modified in the interim.
Offset param is used to track last finished scan.
Mandatory first parameter is a directory with an optional unix style file
filter. Mandatory second parameter
is the name of an adaptor to start.
If the specified directory does not exist, the DirTailer will continue
running, and will start tailing if the directory is later created.
Fields inherited from class org.apache.hadoop.chukwa.datacollection.adaptor.AbstractAdaptor