Expand ID range to harden brute forcing

[cgzones]

Expand the value space from 2^32 to 2^64, resulting in keys of length 11 instead of 6. In particular important for one-time-pastes. i64 is used as inner type since sqlite does not support storing u64.

[matze]

If I see correctly, this will break all existing paste ids?

[cgzones]

Added backwards compatibility.

[matze]

Thanks for the inspiration, I took some of the ideas for 89b215d53309f8fd1632ec4914a023e692b1c716 and credited this prior contribution of yours. Hope that is okay and that I close this PR as being done.