Consider if counters can be switched to Bigint

[drn05r]

It seems as though it is possible for triple and possibly in the future access table to exceed the 2^31-1 limit for standard MySQL ints. It should be investigated whether existing tables can be altered to use bigint instead. This is certainly no bad idea for new archives but if a table reaches such a high ID it is liable to have a lot of records, so any alter table command is likely to take a long time and quite a lot of memory.