Trying to invite multiple people often results in an error

Summary: That is, put multiple address into the "invite by email" space in Sharing. You will often (although not always) get an error, and the invitation will fail.

Actually originally reported by Tara

The problem was a race condition in the database case -- the transaction that was supposed to be working for OID.next() was failing. (Because it was only applying a read-lock to the critical bit, so it was possible for two threads to ask for the current OID before one actually incremented.) As a result, the system would try to create two Identities with the same OID, and boom.

Decided that, rather than trying to patch OID.next() to work correctly, it's time to just dump that and switch to the more-reliable Cluster OID Allocation mechanism that the rest of the system uses. This means that the old System Shard is now completely dead, and not being used for anything any more.