GC for AsOf joins #2010

Consider an asof-join query T1 ASOF JOIN T2 MATCH_CONDITION(T1.ts1 >= T2.ts2), where ts1 and ts2 are timestamp columns in T1 and T2 with LATENESS values L1 and L2 respectively.

Let TS1 and TS2 be the latest observed timestamps in T1 and T2.

Let WL = min(TS1 - L1, TS2 - L2), i.e., the timestamp before which neither of the two streams can see any updates.

• We can GC T1 using integrate_trace_retain_values and the waterline value equal to WL.

• To GC the right side, we need the mechanism described in RFC: Generalizing the GC API to support group operators #1975. Specifically, we need to preserve the latest value preceding WL in each group and all newer values.