Cache contact look up in contact resource.

[hodgestar]

Currently the Javascript sandbox does an index query for every contact look up. We should cache the lookup of the contact key from the address in Redis so that we can avoid the index query.

[hodgestar]

Not completely convinced that this is our bottleneck. Trying praekeltfoundation/vumi#947 first.

[hodgestar]

Closing this for now as the static_reply application seems to have similar performance issues and it doesn't look up contacts at all.

[hodgestar]

It looks like this might still be worth pursuing. Static reply doesn't perform great, but dialogues with contact look ups do still cause Riak a lot more trouble. Re-opening for consideration.