mutation_partition: accurately account for result size in digest only queries
mutation_partition: accurately account for result size in digest only queries

When measuring_output_stream is used to calculate result's element size
it incorrectly takes into account not only serialized element size, but
a placeholder that ser::qr_partition__rows/qr_partition__static_row__cells
constructors puts in the beginning. Fix it by taking starting point in a
stream before element serialization and subtracting it afterwords.

Fixes #3755

Message-Id: <f31b7c80de5ed7b1763a451dbf16be15d0b9d82a@scylladb.com>

avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla