WIP: index: actually reindex when out of order

problem: the out-of-order mechanism based on the outOfOrderIndexerLoop
was not working for some claims.

Let C be a delete claim on permanode P. If C was received before P was,
C was marked as being received with the "have" index row. However, for
the deletion to be marked in the index, some information about P is
needed (its meta row), so C could not be fully indexed upon reception.
Then, when P was finally received, the outOfOrderIndexerLoop would kick
in and retry indexing C. Which would fail, because a test based on the
"have" row would (wrongly) detect that C is already indexed and return
early.

In this patch:

-we introduce the "|indexed" suffix to the "have" row (receive.go). If a
blob is received but some of its dependencies are missing, the have row
is written without the suffix. Upon reception of a blob, we now test for
the presence of the suffix in the have row. If missing, the reception
continues instead of returning early.
The existing mechanism that was detecting missing dependencies for file
blobs has been adapted to work with this suffix too.

-the index enumeration (enumstat.go), which relies on "have" rows, has
been adapted to work with the new "have" row format, while staying
compatible with the old format. And related tests have been added.

http://camlistore.org/issue/454

Change-Id: I2559d08a12b2a4e0f0691fc7e31f1ed1f874625e