Don't do unnecessary index iteration

[johnnason]

Can cause an underlying panic in mgo, and does not seem to serve any purpose.

Connects to https://github.com/compose/transporter/issues/478

Required for all PRs:

• [ ] CHANGELOG.md updated (feel free to wait until changes have been reviewed by a maintainer)
• [ ] README.md updated (if needed)

[codecov[bot]]

Codecov Report

Merging #479 into master will increase coverage by 0.41%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #479      +/-   ##
==========================================
+ Coverage   85.92%   86.34%   +0.41%     
==========================================
  Files          61       61              
  Lines        3587     3580       -7     
==========================================
+ Hits         3082     3091       +9     
+ Misses        334      320      -14     
+ Partials      171      169       -2

Impacted Files	Coverage Δ
adaptor/mongodb/reader.go	86% <100%> (+0.97%)	:arrow_up:
adaptor/postgres/tailer.go	79.32% <0%> (-2.8%)	:arrow_down:
pipeline/node.go	88.46% <0%> (+0.51%)	:arrow_up:
adaptor/elasticsearch/clients/v5/writer.go	88.65% <0%> (+8.24%)	:arrow_up:
adaptor/elasticsearch/clients/v2/writer.go	86.07% <0%> (+10.12%)	:arrow_up:

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[davissp14]

Just to provide some context as to why panics happen here.

https://github.com/go-mgo/mgo/blob/v2/session.go#L1535-L1558

The logic here doesn't account for 64-bit numbers which can be added via https://docs.mongodb.com/manual/core/shell-types/#numberlong