Persistence of completed_requests in sessionmap (do we need it?)

[John Spray <john.spray@redhat.com>]

Zheng noticed on my new sessionmap code [1] that sessions weren't 
getting dirtied on trim_completed_requests.  I had missed that, because 
I was only updating the places that we already incremented the 
sessionmap version while modifying something.

I went and looked at how this worked in the existing code, and it 
appears that we don't actually bother persisting updates to the 
sessionmap if completed_requests is the only thing that changed.  We 
would *tend* to persist it as a consequence to other session updates 
like prealloc_inos, but if one is simply issuing lots of metadata 
updates to existing files in a loop, the sessionmap never gets written 
back (even when expiring log segments).

During replay, we rebuild completed_requests from EMetaBlob::replay, and 
we've made it this far without reliably persisting it in sessionmap, so 
I wonder if we ever needed to save this at all? Thoughts?

Cheers,
John

1. https://github.com/ceph/ceph/pull/3718