Avoid unnecessary codec operations on in-memory subjournals.

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diff --cc manifest
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--- 1/manifest
--- 2/manifest
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@@@ -1,5 -1,5 +1,5 @@@
- C Fix\san\sobscure\sassertion\sfault\sthat\scan\sfollow\san\sOOM.\s\s\sThe\sproblem\nwas\sintroduced\sby\scheck-in\s[a1cf44763277b6c7].
- D 2017-05-06T18:09:36.744
 -C Do\snot\sinvoke\scodec\smacros\sfor\sin-memory\ssubjournals.
 -D 2017-05-10T12:49:50.041
++C Avoid\sunnecessary\scodec\soperations\son\sin-memory\ssubjournals.
++D 2017-05-10T12:58:34.110
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diff --cc manifest.uuid
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--- 2/manifest.uuid
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