kvs-watch: Add flag to only respond to changes to a key

[chu11]

While working on #1653, I realized that the new kvs-watch responds whenever a key has been written to. This includes when the key has the same value written to it. For example:

flux kvs put a=1
flux kvs put a=1
flux kvs put a=1
flux kvs put a=1

Would respond with 4 results to a watcher. In a number of circumstances, I doubt this is what is desired.

Once #1653 is completed, it would be trivial to add a flag to say "only respond if the key value has changed",

[garlick]

I agree that would be useful to have as a separate flag.

[garlick]

FLUX_KVS_WATCH_UNIQ?

[chu11]

FLUX_KVS_WATCH_UNIQ

Actually, this sounds pretty good. B/c it sort of maps to the uniq command and eliminates "repeated lines".

[grondo]

Just thinking aloud again with a focus on typical use cases --should UNIQ be the default behavior?

[chu11]

I thought about that last night. I figure the majority of the time, duplicate values won't be written to the same key, so we probably don't want to introduce a memcmp() into the middle of that code path?

[grondo]

Yeah, I don't know the right answer, but just wanted to bring it up.

[garlick]

I don't have a strong opinion on that one, though I would tend to think the extra overhead is not justified when producer and consumer of KVS data are coordinated (like event logs for example).