Add flag to struct task to mark the task as requiring network epoch.

When processing a taskqueue and a task has associated epoch, then
enter for duration of the task. If consecutive tasks belong to the
same epoch, batch them. Now we are talking about the network epoch
only.

Shrink the ta_priority size to 8-bits. No current consumers use
a priority that won't fit into 8 bits. Also complexity of
taskqueue_enqueue() is a square of maximum value of priority, so
we unlikely ever want to go over UCHAR_MAX here.

Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D23518