net: tcp: Make priority of TCP work queue configurable

The TCP work queue is handles all TCP transmission and maintenance tasks.
Make it's priority configurable, so it can be configured to a lower
priority than the lower level network layers to avoid it consuming all
net_bufs before handing over execution to the lower layer network layers.

Signed-off-by: Sjors Hettinga <s.a.hettinga@gmail.com>