This patch adds the _static_shares variable to the backlog_controller so that
instead of having to use a separate constructor when controller is disabled,
we can use a single constructor and periodically check on the adjust method
if we should use the static shares or the controller. This will be useful on
the next patches to make compaction_static_shares and memtable_flush_static_shares
live updateable.
Signed-off-by: Igor Ribeiro Barbosa Duarte <igor.duarte@scylladb.com>