`Conn.AddTable` use netlink.Create which will not emit an error
if the table we want to create already existed,
just like the `nft add table ...` command works.
The caller should use netlink.Excl to
get an EEXIST error for that already existed,
So I add another method `Conn.CreateTable`
which works just like `nft create table ...` command.
Related: #245
Signed-off-by: black-desk <me@black-desk.cn>
* Close receiver for lasting netlink connections while defaulting to existing temporary netlink connection usage
* add unit test for New lasting connection, Close and correct default connection handling behavior
* refactor tests to use New constructor
* make Conn mutex un-exported (#159)
fixes issue #157