increase size for unix domain socket pathname from arbitrary 100
to PATH_MAX to solve tests erroring out on OSX with long temporary
file names.
work towards converting rhizome fetch logic to use new
rhizome_database_open_blob() and friends. #49
rhizome_open_write() now uses rhizome_database_create_blob_for()
to find rowid of blob, and thus allow use of external blob files.
rhizome_store.c bundle access functions now use
rhizome_database_blob_*() functions, thus allowing use of external
blob files. #49