mds: match last snap exactly on replay, add_*_dentry

mds: match last snap exactly on replay, add_*_dentry

In general, we add new snapped dentries and THEN the new live dentry
to the metablob.  That means that during replay, we see [2,2] followed
by [3,head], replacing [2,head].  The [2,2] dentry should be added
anew, without paying heed to [2,head], and then the [3,head] should
replace/update [2,head].

It was mainly just the assertions in add_*_dentry that were getting
in the way.. but the lookup_exact_snap is also slightly faster.