CADC: cache status of e-transfer files via os.walk

Now use os.walk to search all of the e-transfer directories.  The
status of each file is now stored in a cache dictionary to allow
multiple queries to be made in an efficient manner.
CADC: complete implementation of e-transfer poll
test: Correct reference ACSIS data directory
CADC: add a warning to the e-transfer poll routine

This warning will be logged if the files have left the e-transfer
directories but do not appear to be at CADC.
CADC: add additional file search patterns

The CADCFiles class needs these file search patterns in order
to know how to query the JCMT info service.