Comment 4 for bug 1643024

So, the reason is that we have very long transaction (for the whole graph serialization) where statements like:

UPDATE nodes SET primary_tags=ARRAY['controller'] WHERE nodes.id = 1

block simultaneous updates of node timestamps by nailgun agent:

UPDATE nodes SET timestamp='2016-12-09T05:54:08.388083'::timestamp WHERE nodes.id = 1

At some point this results in all web server workers being blocked waiting for DB to complete the transaction.