p4fastimport: only disable LFS uploads if p4fastimport.lfsmetadata is set
Currently, normal LFS uploads are completely disabled during a p4fastimport,
and users must run an external SQLite-reading uploader app after the import.
Instead, allow normal LFS functionality to work, but users may override it to
do the SQLite thing.
Differential Revision: https://phab.mercurial-scm.org/D1307