journal: extract journal_write_row from limbo

The function writes a single xrow into the journal in a blocking
way. It isn't so simple, so makes sense to keep as a function,
especially given that it will be used more in the next commit.

Part of #10113

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

(cherry picked from commit 7d10096c)