- Updated the orderer's file ledger to use it's underlying
  blockstore ledger impl's iterator APIs.
- Blockstore iterators must be explicitly closed to avoid
  leaking resources, so now orderer/ledger.Iterator must
  also be explicitly closed. Added Close() to the
  orderer/ledger.Iterator interface.

Change-Id: Id838a661a11bf5b64a0cbb57d75a27d69d251269
Signed-off-by: Luis Sanchez <sanchezl@us.ibm.com>