perf(plook): cache available versions when possible

To speed up version lookup, we'll cache them together with the
(already cached) slug. This enables for a quicker resolution of available
versions.
perf(plook): use custom HTTP agent

By using a custom HTTP agent, we are able to increase the maxSockets.
This allows for much greater performance when doing ApacheBench
tests, for example.
Merge branch 'perf-enhancements'