streamchain

This change set makes a failure pulling the system chain to panic.

Tests have been modified to accomodate:

- TestReplicateChainsFailures in replication_test now panics in all cases
  because it invokes ReplicateChains which invokes PullChannel indirectly.

- TestReplicateIfNeeded in onboarding_test now panics in the last test case
  because all we care about (to test) is the fact that we successfully attempt
  to pull from the orderer, and not whether the blocks actually arive from it,
  which is tested in the cluster tests.

Change-Id: Ied21201e693536935eaa708dc3692b76a49d1c61
Signed-off-by: yacovm <yacovm@il.ibm.com>