Fix missing return and Mongo3 support for index_miss_ratio option

[olivierHa]

Hello,

In Mongo3, the server status output no longer includes the workingSet, indexCounters, and recordStats sections.

So the index_miss_ratio function is broken.

See : http://docs.mongodb.org/manual/reference/command/serverStatus/#output

This patch disable this action when using mongo3 (similar to the lock action) and fix the return code.

Olivier