Ravendb 1404 - Safer NextIndexId

So I decided to use the incrementing id code that already exists. (It seems other things are using it too so that seems okay).

I updated the 47 to 48 migration script to set this seed value (Making the assumption that this is un-released software (including the id changes already made) and it doesn't require another migration script/version).

I tested this with the code I wrote to test migrations originally and the seed appears to work correctly with new indexes.