Keep the highest _id given out in Fbe.fb so a deleted one is never reused

[Thayorns]

Fbe.fb numbered every new fact from (max _id) of the facts that were still in the factbase. Once the fact with the highest _id was deleted, the next insert got its number again, so anything that kept that _id from earlier in the run, such as Fbe.overwrite, Fbe.delete or the log lines, pointed at a different fact.

The Pre block now remembers, in a local it closes over, the highest _id it has given out while the cached factbase lives, and takes the larger of that and (max _id), so a number is never handed out twice. The (max _id) part stays, so a factbase that arrives with facts already numbered still continues after its highest one.

The tests delete the last fact, delete every fact, and delete then insert inside one transaction, and each checks the next _id. One more test makes sure numbering still follows the highest _id of the origin factbase.

This covers one run. An _id deleted in a previous run and not saved into the factbase can still come back in the next one, because nothing records it; that would need the counter persisted somewhere, which is a bigger change than this issue.

Closes #1245

[Thayorns]

@yegor256 take a look please, happy to clarify anything about the change.