kiji-rest: REST-77. Updates load tests.

review: https://review.kiji.org/r/1397
issue: https://jira.kiji.org/REST-77
commit: https://github.com/kijiproject/kiji-rest/commit/ec3dcbf7077f88a748c438c1fe8093b319824875
kiji-rest: REST-50. Fix a performance issue that was causing massive slowdowns.

In order to fix the issue about checking whether or not a table is valid, the
initial implementation checked the metatable for each request which was causing
a major slowdown. The solution now is to let the first request to a deleted
table fail and then remove that table for future use so that future calls will
return a 404 instead of hanging and throwing a 500.

Reviewer: lsheng
commit: https://github.com/kijiproject/kiji-rest/commit/2e1f59395bb64c452c645434066b029f737b2264