[CBM] Merge/restore fails when the last mutation in the first backup is in a collection deleted in the second

Reproduction

1. Create two collections, c1 and c2
2. Create a mutation to key 'A' in c1 and c2
3. Do a backup
4. Create a mutation 'B' in c1 and c2
5. Delete c2
6. Do a backup
7. Do a restore/merge

What should happen

The merge or restore should complete and contain just 'A' and 'B' in c1

What actually happens

The merge/restore correctly transfers data from the first backup but during the second backup fails with a message such as:

Error merging data: snapshot marker for vBucket 55 isn't valid {Timestamp:0 Start:0 End:5} -> {Timestamp:0 Start:4 End:6} 

During a merge/restore we only transfer the data that will exist at the end, so if you delete a collection the mutations from it won't be transferred. We therefore advance the seqno to take account of any mutations in a deleted collection that happen after the last transferred mutation. Up to here the behaviour is correct. At the end of a transfer however we will flush the writer and then persist the last committed seqno - but this can be lower than the seqno we've advanced forward.

In the example above we have the following mutations:

backup 1:

4 - c1 A

5 - c2 A

backup 2:

6 - delete c1 

 so here we will advance to seqno 5 to take account of the creation of A in c2, but the last committed seqno will be 4 as it is the last mutation in an alive collection - ie one that we transfer.