fix growing allocation of pendingOffsets when no offsetManager is in use

[jipperinbham]

fixes #434

[jipperinbham]

for reference, before the fix:

(pprof) top --cum
284.31MB of 284.31MB total (  100%)
Showing top 10 nodes out of 11 (cum >= 13.50MB)
      flat  flat%   sum%        cum   cum%
         0     0%     0%   284.31MB   100%  runtime.goexit
  132.80MB 46.71% 46.71%   270.81MB 95.25%  github.com/compose/transporter/pipeline.(*Node).write

after the fix:

(pprof) top --cum
201.02MB of 201.02MB total (  100%)
Showing top 10 nodes out of 11 (cum >= 16.50MB)
      flat  flat%   sum%        cum   cum%
         0     0%     0%   201.02MB   100%  runtime.goexit
   47.50MB 23.63% 23.63%   184.51MB 91.79%  github.com/compose/transporter/pipeline.(*Node).write

[codecov]

Codecov Report

Merging #436 into master will decrease coverage by 0.13%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #436      +/-   ##
==========================================
- Coverage   84.17%   84.03%   -0.14%     
==========================================
  Files          61       61              
  Lines        4182     4184       +2     
==========================================
- Hits         3520     3516       -4     
- Misses        492      496       +4     
- Partials      170      172       +2

Impacted Files	Coverage Δ
pipeline/node.go	86.03% <100%> (+0.06%)	⬆️
adaptor/postgres/tailer.go	78.21% <0%> (-2.98%)	⬇️

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