net: tcp: Don't send active close FIN packets synchronously

A FIN packet generated by a net_context_put() must go into the normal
transmit queue and not be sent synchronously.  Previously sent data is
expected to be delivered and acknowledged before the connection is
terminated.

An advantage we get with this change is unified timeout and retry
handling for FIN packets.

Note that there remains a misfeature here where the queing of the FIN
results in a synchronous switching of the connection callback to
tcp_active_close(), which will prevent any further data received from
being provided to the user.

Change-Id: I2d41316549da9fee383b4f32af5e8b3adf4cb122
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>