Release a stamped reservation the commit did not condense

prepare-commit-msg reserves the stamped checkpoint on each session, but only a successful condense cleared it. A session stamped yet not condensed kept the reservation, so its next commit reused the ID, stamping a second commit with the first one's checkpoint, and stampedByAnotherCommit exempted exactly that reuse. The reservation is now marked as stamped-only (a condensation that starts writing replaces it), and post-commit releases it on every session it did not condense. Also drop a dead ErrMutationSkip check.