perf(cluster): improve the performance of calculating slots

[luin]

By using the https://www.npmjs.com/package/cluster-key-slot package.

[shaharmor]

LGTM

[Salakar]

Just one small change that needs doing, the pipeline can use var calculateSlotMulti = require('cluster-key-slot').generateMulti; to calculate the slots for the array of keys there, then instead of checking for 'undefined' you just check if slot < 0 (returns -1 if any slot isn't the same) - then throw the 'keys must be of same slot' error.

The multi on the calc repo is a little bit more performant. https://github.com/Salakar/cluster-key-slot#usage

[Salakar]

Hmm build test failures seem a little random and unrelated?

[luin]

@Salakar Just fixed it. If one of the test failed, the connection to the server will not be disconnected, so other tests will also fail.

[Salakar]

Looks good, LGTM =]