Port the PersistentSymbolTable to LockedItemRepository::op

This is seemingly less obvious, but thankfully actually totally fine.
I was first very surprised by how the code used the same request
to insert a new changed item, but that relied on the reference-holding
semantics of the request. To make this more clear, get rid of the
requests and operate on items directly. Then remove the external
mutex locking and only lock the item repo mutex when needed via
the LockedItemRepository::op. We also need a separate mutex to ensure
the caches are properly protected, because they are accessed outside
the repository ops.

In general, I think this whole class needs a cleanup, but that will
have to wait for a future time.