Read doc from new_file in iterator, during compaction

- During compaction, iterator's WAL tree is organized on new file,
so that its items contain byte offsets of corresponding documents
in new file. Thus, iterator functions have to read the documents
using new_file's docio_handle.
Merge remote-tracking branch 'origin/1.0b'

* origin/1.0b:
  Read doc from new_file in iterator, during compaction

Conflicts:
	src/iterator.cc

Change-Id: I228a2b2b4c38de1103dc0d7f7b8d3e51ec83facb