mutation_partition: make rows_entry constructors explicit
mutation_partition: make rows_entry constructors explicit

All converting constructors should be explicit otherwise they
can create a confusion. I got myself in such a situation when
clustering key got implicitly converted into rows_entry when
I was not expecting it.

Signed-off-by: Piotr Jastrzebski <2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>
Message-Id: <c3f19719760f6dc7cf5e858b9c452506faedf521.1485950529.git.2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>

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