MB-14637 pipeline: Add PeekBlock() and FlushBlock() APIs
MB-14637 pipeline: Add PeekBlock() and FlushBlock() APIs

We may need to read entire items at one shot without reading, items
one by one. PeekBlock() can be used to read full block at one shot.
If a PeekBlock() is used, user should invoke FlushBlock() after
peeked block is not used anymore.

Change-Id: I367331340bc0e2fe5479d3aa861cc2a723b6a31f

jchris/indexing,jchris/indexing,jchris/indexing,jchris/indexing