Add function for getting current wallclock time about arroyo HOT 1 OPEN

Thanks for reporting. There are at least four possible definitions for "current time" in arroyo

1. The time the query is started (which is now() in Arroyo)
2. The current clock time
3. The event time of the row it's called on
4. The current watermark time

Currently arroyo just implements the first one (via now()), but others would be useful as well. The second one is easy to get in a UDF today, but the other two would need to be built in.

In Flink, now() is the current clock time (2) in streaming mode, but query start time (1) in batch mode. We might want to adopt that behavior, but I think the current version of now() is also useful in some cases and is worth preserving. Adding functions for 3 and 4 would also be great (in Flink 4 is CURRENT_WATERMARK which seems like a fine name).

Writing new functions is pretty straightforward so this could be a great first issue for someone.

from arroyo.