Optimizations to improve flow setup rate
- Fetch UTCtimestamp only at the beigning of flow_handler proto process
  i.e., max upto 32 entries can have same setup time.
- get ksync nexthop from DB state instead of lookup
- Synchronous KSync-Vrouter operations, to remove limitation of 200
  outstanding message (for faster processing.)
Need to consider multiple response to one request.
Taking care of review comments.
    - added a knob in KSyncSock to run as Sync or Async
    - reverted changes for UTCtimestamp, will take it up later.
For link local service request, do src port nat. Bind to a local port, and use it as nat src port.
Update test case
Merge remote branch 'upstream/master'
Update for code review
Merge remote branch 'upstream/master'

Conflicts:
	src/vnsw/agent/pkt/flow_table.h
	src/vnsw/agent/pkt/test/test_pkt_flow.cc
Merge remote branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
fixing test cases for ksync running in sync mode.
adding test-case to simulate failure for get nh from DB state
Adding test case for KSync running in async mode.
Changes for flow message processing optimizations in collector:
1. Rewrite FlowTableInsert to traverse the flow message XML
  tree only once and extract the needed fields into an
  array. Use the array to populate the vectors needed to
  populate the FlowRecordTable and the flow index tables.
2. Use the DbDataValueType type to encode the value into
  string rather than using the column family name to determine
  the type. This avoid 2 map lookups per field encoded.
  Add a boost::blank as the first element of DbDataValueType
  variant instead of std::string to determine if the variant
  is populated or not.
3. Add a mock_generator in C++ which sends flow messages and
  can be used to scale test vizd.
4. Use pugixml to decode the SandeshHeader and the sandesh
  message in SandeshServer. Abstract out SandeshMessage
  creation and parsing using SandeshMessageBuilder, change
  vizd to use SandeshMessage.

With the above changes, we are able to increase vizd flow sandesh message
receive performance for 1 generator from around 1.5K msg/sec to
around 3.5K msg/sec using vizd built using compiler optimizations enabled -03
on single box setup - a6s23 - 4 core i3.
Enable udp_io_test in test suite, wrongly commented out
Fix concurrency task failure for test_nh
Merge pull request #348 from Juniper/meghb_flow

Changes for flow message processing optimizations in collector:
Limit the number of linklocal flows
Merge remote branch 'upstream/master'
Merge pull request #350 from manishsing/master

Fix concurrency task failure for test_nh
Rename kMaxOpenFiles to kMaxOtherOpenFds