OFStateManager: move ptr_cxn_wrapper_t to the internal header

ofstatemanager.c needs this.
OFStateManager: fix bug when marking for deletion

ft_entry_mark_deleted always set the state to DELETE_MARKED whether or not the
flow was stable, so we'd always make the call into forwarding immediately (even
if the previous state was MODIFYING, meaning we'd have two requests in flight
for the same flow!).

The fix is to only send the call to forwarding if the flow was stable before
marking it for deletion.
OFStateManager: fix barrier tracking for flow deletes

Previously, the delete message handlers would free the LOCI message immediately
without waiting for Forwarding to signal that the deletion was complete. This
meant pending deletions would not block a barrier.

This change passes the usual ptr_cxn_wrapper_t as the cookie to
indigo_fwd_flow_delete instead of just the entry pointer. The ptr_cxn retains
the LOCI request for barrier tracking.

The queued request code is changed significantly. Previously (contrary to the
comments) a deletion was not tracked on the queued_reqs list. Now the ptr_cxn
is enqueued and any existing queued modifies are deleted.
OFStateManager: document state transitions for flow entries
Merge into master from pull request #39:
Fix barrier tracking for flow deletion (https://github.com/floodlight/indigo/pull/39)