sstables::write_simple() has quite a lot of boilerplate
which gets replicated into each template instance. Move
all of that into a non-template do_write_simple(), leaving
only things that truly depend on the component being written
in the template, and encapsulating them with a
noncopyable_function.
An explicit template instantiation was added, since this
is used in a header file. Before, it likely worked by
accident and stopped working when the template became
small enough to inline.
Tests: unit (dev)
Message-Id: <20200106135453.1634311-1-avi@scylladb.com>
e5e42672f5