* core: lock delivery and relay request work items on PostgreSQL
* core: retry store transactions aborted by PostgreSQL deadlocks
* core: show skipped messages followed by agent messages
* core: read delivery and relay request work items in one snapshot
* core: use the conflict retry of the store transaction for work item reads
* core: show integrity errors of agent messages from MSG without body
* core: show integrity errors from MSGERR, log MSGERR without an error
* core: update simplexmq, set server forward workers in tests
* core: update simplexmq