COMPASS 171: Fix doc & index stats on secondary reported incorrectly

[pzrq]

Better fixes incoming in COMPASS-368 and later tickets.

[pzrq]

Going to squash, merge and publish this bug fix as it needs to go upstream into data-service and then onwards to Compass. If it somehow causes other unintended issues we can either revert it or roll forward.

Note: @KeyboardTsundoku No tests due to technical debt in this area because of things like node v5 and mongodb-runner being very out of date. e.g. npm install in mongodb-runner generates the following warnings that might be familiar to you:

pzrq@rocket:~/Projects/runner$ npm install
npm WARN deprecated graceful-fs@2.0.3: graceful-fs v3.0.0 and before will fail on node releases >= v7.0. Please update to graceful-fs@^4.0.0 as soon as possible. Use 'npm ls graceful-fs' to find it in the tree.
npm WARN deprecated minimatch@2.0.10: Please update to minimatch 3.0.2 or higher to avoid a RegExp DoS issue
npm WARN deprecated minimatch@0.2.14: Please update to minimatch 3.0.2 or higher to avoid a RegExp DoS issue
npm WARN deprecated graceful-fs@1.2.3: graceful-fs v3.0.0 and before will fail on node releases >= v7.0. Please update to graceful-fs@^4.0.0 as soon as possible. Use 'npm ls graceful-fs' to find it in the tree.
npm WARN deprecated minimatch@0.3.0: Please update to minimatch 3.0.2 or higher to avoid a RegExp DoS issue
npm WARN deprecated lodash.assign@4.2.0: This package is deprecated. Use Object.assign.