Fix next_id generation for new records

[stex]

ProxyCache#next_id should create an identifier for a record created in the current transaction which was not used before.

It currently increments an identifier of the format new_\d+ which leads to problems in the following case:

1. create a new record
2. delete this record
3. create another record => it will receive the same identifier

We should most likely be able to simply use SecureRandom for identifier generation and just check for existence.