Move context of CacheFetchProc from afs_dcache.c to afs_fetchstore.c

Move context of CacheFetchProc from afs_dcache.c to afs_fetchstore.c

GetDCache() is quite bloated. This inlinining makes code more
readable. For protocols to come besides rxfs (such as rxosd), some
initializations may need to be performed differently. Thus, much
has to be moved to the specific rxfs_fetchInit() function rather than
afs_CacheFetchProc() proper.

Reviewed-on: http://gerrit.openafs.org/112
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>