streamed_mutation: do not call fill_buffer() ahead of time
streamed_mutation: do not call fill_buffer() ahead of time

consume_mutation_fragments_until() allows consuming mutation fragments
until a specified condition happens. This patch reorganises its
implementation so that we avoid situations when fill_buffer() is called
with stop condition being true.
Message-Id: <a1b89f9249d0ec724f0a88b4a8e04c9d18d022bb@scylladb.com>

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