We want to make table::_truncated_at optional, so that in
get_truncation_time we can assert that it is initialized.
For existing tables this initialisation will happen in
load_truncation_times function, and for new tables we
want to initialize it in add_column_family like we do
with mark_ready_for_writes.
Now add_column_family function has parameter 'readonly', which is
set by the callers to false if we are creating a fresh new table
and not loading it from sstables. In this commit we rename this
parameter to is_new and invert the passed values.
This will allow us in the next commit to initialize _truncated_at field
for new tables.
32a19fd61b