server: remove unreferenced non-root objects

[jcaamano]

From rfc 7047:

      The "isRoot" boolean is used to determine whether rows in the
      table require strong references from other rows to avoid garbage
      collection.  (See the discussion of "strong" and "weak" references
      below in the description of <base-type>.)  If "isRoot" is
      specified as true, then rows in the table exist independent of any
      references (they can be thought of as part of the "root set" in a
      garbage collector).  If "isRoot" is omitted or specified as false,
      then any given row in the table may exist only when there is at
      least one reference to it, with refType "strong", from a different
      row (in the same table or a different table).  This is a
      "deferred" action: unreferenced rows in the table are deleted just
      before transaction commit.

[dave-tucker]

this is on my todo list, i'll hopefully get to it next week.

i'm not 100% sure how to implement the reference counter at the moment, but this is my plan.

1. map[string]map[string]map[string]int is our ref counter, which maps database name, to table, to uuid, to count 😓
2. when we create a non-root object we add it to the map
3. when something references that object we increment the counter (note: we should also enforce strong vs weak refType checking here too)
4. when something removes a reference to that object we decrement the counter, if it's zero we add it to another map to flag it for garbage collection
5. before Commit is called at the end of the transaction, we remove any items from the garbage collection map from the database (which could include rows that we were committing as part of the transaction)

[jcaamano]

When pondering about this I toyed with the idea of placing the ref counter as a hidden column somewhere in the existing rows. Other than that I had the same ideas.