kvs: add FLUX_KVS_PEDANTIC flag for lookups

[garlick]

As mentioned in #1556, the proposed lightweight KVS watch has semantic differences from the current KVS watch service, arising from fact that, in the lightweight implementation, the the key is only looked up when it is mentioned verbatim in a kvs.setroot event (which tracks the keys mentioned in a KVS transaction). Other actions that could affect the value that would be looked up are not detected, such as replacing its parent directory.

The current KVS watch service is heavyweight because it checks the value of each watched key on each kvs.setroot event.

Although the heavyweight design may not be desirable in the common case, there may still be cases where it is useful. Furthermore, existing tests, bindings, etc.. depend on these semantics for the moment.

To implement a flag which activates the heavyweight semantics, the kvs-watch module would need two changes

1. ignore the key list in the kvs.setroot event and look up the key on every commit
2. track the last value returned to the user, and return a response only when it has changed