flow: improve 'under stress' behavior

flow: improve 'under stress' behavior

When under stress, the packet threads ultimately fall back
to walking the hash table until they find a flow they can
safely evict and reuse. This could lead to all threads
fighting over the FlowBucket locks.

Fix by adding a limit to the number of hash rows that are
checked for a new flow. If the limit is reached, simply fail
to get a flow.