interimap: raise SELECT sample set size from 64 to 256 bytes.

A n-bytes set covers at least ⌊(n+1)/11⌋ UIDs (UIDs are at most 10 bytes
of size), hence 23 UIDs for 256 bytes long sets.

However we exceed it by another range, so in the worst case (if the the
higher UIDs are sparse) we'll sample ⌊(n+1)/11+1⌋ UIDs:

    1000000000,1000000002,1000000004,…,1000000046

This was 6 UIDs for n=64 which is a tad low; this is now raised to 24
UIDs.  The actual set size returned by sample() is of max size n+22
bytes (extra "$UID1:$UID2," where $UID1 and $UID2 are both ≥10⁹).

0 files changed, 0 insertions, 0 deletions