Scenario:

* A: new PouchDB() called
* A: afterDBCreated() called, updateSeq initialized to 0
* B: db.put() called
* A: fetches current updateSeq from the database
* B: writeDoc() called, updateSeq++, updateSeq=1 put in database
* A: callback to get(updateSeq), overwrite updateSeq to whatever was in the db (e.g. 21)

Result: updateSeq is 21, when it should be 22.

I think we will have to use the same solution to `docCount` here, i.e. a task queue, unless I'm mistaken.