OFStateManager: add function to lookup gentable entry by key

This change ports lookup functionality from the group table to gentables. This
is useful for when one gentable needs to refer to another, removing the need to
create a separate hashtable.
OFStateManager: add APIs to acquire/release gentable entries

These functions are analogous to the ones defined for the group table. As long
as a reference is held to a gentable entry it can't be deleted, so the private
data pointer remains valid. This allows the referencing code to store the
private data pointer in its own datastructures without risking a dangling
pointer error.
OFConnectionManager: allow blocking barriers on nonexistent connections

I've seen this case in IVS when a flow-delete long running task outlives the
connection that spawned it.
Merge into master from pull request #175:
OFConnectionManager: allow blocking barriers on nonexistent connections (https://github.com/floodlight/indigo/pull/175)
Merge into master from pull request #174:
Port lookup/refcount functionality from group table to gentables (https://github.com/floodlight/indigo/pull/174)
OFStateManager: convert list iteration to of_object_t
update bigcode

* submodules/bigcode 431fd61...6834d73 (4):
  > Merge into master from pull request #38: Editline Server Module. (https://github.com/floodlight/bigcode/pull/38)
  > Merge into master from pull request #37: slot allocator module (https://github.com/floodlight/bigcode/pull/37)
  > Merge into master from pull request #36: Latest (https://github.com/floodlight/bigcode/pull/36)
  > Merge into master from pull request #35: Add burst parameter to pimu_flow_pps_set. (https://github.com/floodlight/bigcode/pull/35)
update infra

* submodules/infra 3ffc4c1...b4d775f (13):
  > Add 8bits datatype.
  > Merge into master from pull request #31: Fix obj ratelimit macro (https://github.com/floodlight/infra/pull/31)
  > Merge into master from pull request #28: add update-submodules.sh script (https://github.com/floodlight/infra/pull/28)
  > Merge into master from pull request #30: AIM logging function (https://github.com/floodlight/infra/pull/30)
  > Merge into master from pull request #27: Introduce the DEPENDMODULE_HEADERS variable. (https://github.com/floodlight/infra/pull/27)
  > Merge into master from pull request #26: AIM: squelch unused variable warnings from ASSERT (https://github.com/floodlight/infra/pull/26)
  > Merge into master from pull request #25: AIM: add wrappers for __attribute__ (https://github.com/floodlight/infra/pull/25)
  > Add aim_bitmap 512.
  > Merge into master from pull request #24: Always update the modulemake file by default. (https://github.com/floodlight/infra/pull/24)
  > Support flag maps. - Enumerations that are defined as flag bits can be registered as a flag map datatype. - Flagmap datatypes are displayed in the following format: 0x%.8x [f0,f1,f2...fn]
  > Merge into master from pull request #23: Support to check if two aim bitmaps have any intersecting bits. (https://github.com/floodlight/infra/pull/23)
  > Merge into master from pull request #21: modify log time format (https://github.com/floodlight/infra/pull/21)
  > Merge into master from pull request #22: Updates for use in shared libraries.  (https://github.com/floodlight/infra/pull/22)
update loxigen-artifacts

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Merge into master from pull request #177:
Convert LOCI list iteration to of_object_t (https://github.com/floodlight/indigo/pull/177)