SocketManager: add task API
SocketManager: sort tasks by priority

Requires a sorted insert on task registration, but removes the need to traverse
the whole list in the main event loop.
OFStateManager: use ft_flow_first_match for strict matches

This makes it clearer which operations can affect multiple flowtable entries.
OFStateManager: inline flows_delete

Only used in one place.
OFStateManager: add interface for iterating over the flowtable in a task
OFStateManager: convert flow delete handler to task