receiver optimisation: defer lookup of xattrs

[mharvey-jt]

Looking at the grafting of a particularly large catalog we have, I see surprisingly large runtimes. The graft is of a sub directory to a directory containing 44k+ directories nested catalogs, no files, and full path length of ~ 34 chars. Each directory has an ACL.

The presence of the ACL causes an xattr lookup

cvmfs/cvmfs/catalog_diff_tool_impl.h

Lines 153 to 155 in 47a223e

	if (new_entry.HasXattrs()) {
	new_catalog_mgr_->LookupXattrs(new_path, &xattrs);
	}

This is slow, and the xattrs are not used in the case I'm looking at. Moving the xattr lookup into the blocks where is it actually used markedly improves runtime - by 6x in this case.

[mharvey-jt]

See #3570