event/dsw: improve migration mechanism

event/dsw: improve migration mechanism

Allowing moving multiple flows in one migration transaction, to
rebalance load more quickly.

Introduce a threshold to avoid migrating flows between ports with very
similar load.

Simplify logic for selecting which flow to migrate. The aim is now to
move flows in such a way that the receiving port is as lightly-loaded
as possible (after receiving the flow), while still migrating enough
flows from the source port to reduce its load. This is essentially how
legacy strategy work as well, but the code is more readable.

Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>