Add random rowid sampling helper - #14

Summary

Adds Cache._sample(n), a helper method that returns n randomly picked sample rows from the Cache table for later scoring for size + access_count

Changes

• Sampling using random rowid jumps instead of using ORDER BY RANDOM() to ensure the cost remains roughly same regardless of the table size.
• Duplicate row ids within one call of _sample are skipped with the help of the seen set.
• Handles the cases where cache could be empty or cache size is less than that of input 'n' to _sample(n)

Testing

• Adds 3 tests, runs successfully, covering the empty cache case, and less number of items than input.
• Complete test-suite passes successfully. Test coverage above 98%.