Fix LDB dumpwal to print the messages as in the file.
Fix LDB dumpwal to print the messages as in the file.

Summary:
StringStream.clear() does not clear the stream. It sets some flags.
Who knew? Fixing that is not printing the stuff again and again.

Test Plan: ran it on a local db

Reviewers: dhruba, emayanke

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D6795

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