indigo: make indigo_fwd_table_stats_get synchronous
indigo: make indigo_fwd_flow_stats_get synchronous

This simplifies flow stats, aggregate stats, and flow expiration.
indigo: make indigo_fwd_flow_create synchronous
indigo: make indigo_fwd_flow_modify synchronous

The queued_req code is now unused and removed.
indigo: make indigo_fwd_flow_delete synchronous

Removes the need to dup the delete message for every flow.
OFStateManager: get rid of flow states

Outside of flow-mod handlers, the flows in OFStateManager and Forwarding are
identical.
OFStateManager: removed flow removal reason from ft_entry

This field is only used in the context of deleting a flow.
OFStateManager: fix outdated comments in ft_entry.h
indigo: remove unused interface indigo_fwd_table_mod
indigo: make indigo_port_modify synchronous
indigo: make indigo_port_stats_get synchronous
indigo: make indigo_port_queue_config_get synchronous
indigo: make indigo_port_queue_stats_get synchronous
indigo: remove comments on synchronous interfaces

All interfaces are synchronous now.
OFStateManager: remove text about callbacks

Also slightly improved the error messages I touched.
OFStateManager: fix bad merge
OFStateManager: remove unused ptr_cxn_wrapper_t
indigo: update indigo_port_queue_stats_get comment
OFStateManager: comment on flow stats iter cleanup
Merge into master from pull request #55:
synchronous Forwarding/PortManager APIs (https://github.com/floodlight/indigo/pull/55)