Reenable gen test

[guybedford]

I had to disable the generation insert test in #1120 because it is consistently failing.

We should investigate the root cause here and reenable the test to be sure the functionality is working correctly.

/cc @vcarvajal-sigsci

[guybedford]

Specifically, kv.lookup() should be able to get the generation integer I think, and then that is the integer that should be used on insertion to avoid double insertion. So we might have a misuse of the API.

Alternatively we may have a KV bug either in our implementation or otherwise that this isn't working correctly - either way it needs to be investigated.

[guybedford]

I think we might need a kv.lookupWithGeneration since the existing lookup doesn't return the generation integer.