When merging statistics buckets for UNION and UNION ALL queries
involving a column that maps to Double (eg: floats, numeric, time
related types), we could end up in an infinite loop. This occurred if
the bucket boundaries that we compared were within a very small value,
defined in Orca as Epsilon. While we considered that two values were
equal if they were within Epsilon, we didn't when computing whether
datum1 < datum2. Therefore we'd get into a situation where a datum
could be both equal to and less than another datum, which the logic
wasn't able to handle.

The fix is to make sure we have a hard boundary of when we consider a
datum less than another datum by including the epsilon logic in all
datum comparisons. Now, 2 datums are equal if they are within epsilon,
but datum1 is less than datum 2 only if datum1 < datum2 - epsilon.

Also add some tests since we didn't have any tests for types that mapped
to Double.