Bluetooth: Host: Run tx processor on its own thread

When thread that TX processor is used to send commands and data to
Controller is also used for sync commands sending and command buffer
allocation, a deadlock happens.

This thread is used to avoid such deadlocks by moving TX processor to
its own dedicated thread exclusively used by tx processor only.

Co-authored-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>
Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>
Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>