also verify oplog doc is for the tailing collection

[jipperinbham]

fixes bug reported via gitter chat where documents form collections not being tailed were making their way down the pipeline

this is possibly a bug in the upstream mgo project or mongodb itself but to ensure it doesn't happen in transporter, we'll also verify the oplog doc ns matches the ns being used for the query

[codecov]

Codecov Report

Merging #386 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #386      +/-   ##
==========================================
+ Coverage   86.33%   86.33%   +<.01%     
==========================================
  Files          59       59              
  Lines        3468     3469       +1     
==========================================
+ Hits         2994     2995       +1     
  Misses        313      313              
  Partials      161      161

Impacted Files	Coverage Δ
adaptor/mongodb/reader.go	85.43% <100%> (+0.07%)	⬆️

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