We switch everything to use QueryContext/QueryRead/etc instead
of Qcx/Tx.
We drop the short_txkey subpackage (it's now handled by either
keys or querycontext).
We drop all the dbshard stuff, and all the tx/txfactory stuff.
We remove all the things that related to the old "Block" concept,
which was mostly used by the anti-entropy code, but had one
fragmentary usage left in the ImportRoaringOverwrite case of
ImportRoaring. That's replaced by using a rewriter that deletes
all bits (not just bits in specific columns) from an existing
thing, but writes in new bits. Actually we could probably do that
better with a custom "eradicate-rewriter" that doesn't try to
be clever, and just eliminates things.
This includes a number of minor bug fixes that were
exposed by getting the testing to work. For example:
* When checking whether an operation "requires write", we
now consider a Delete a kind of a Write, because it is.
* Several tests were relying on the fact that writes through
Qcx were being committed whether or not the Qcx was ever
told to finish. With QueryContext, you actually have to
reach a Commit() or the writes don't happen (except for
special cases in Delete).
* Replaced a lot of panics with t.Fatalf in tests.
There's also some minor staticcheck fixes, like deleting the
unused "db" member of a boltdb transaction wrapper.
* Clean up dax service interfaces
Rename some of the `computer` interfaces and organize them in the
appropriate files.
Remove `dax/computer/alpha` package
* Remove ComputeAPI (it was replaced by batch.Importer)
* add nss-tools dependecy to smoke test
