Step one: switch to etcd.io's bbolt fork of boltdb.
The etcd-io fork of boltdb isn't archived, and has fixes for boltdb's
interactions with checkptr, allowing us to drop the checkptr-disabling
hackery.
This seems to be a drop-in replacement; etcd/bbolt says that the file
format is "fixed" (I believe in the sense of "unchanging"), and I can
run pilosa on an existing data directory with this.
Step two:
Fix missing caps in roaring.go that were also triggering the same
issues.
There's two kinds of unchecked errors here. Writes to a hash
(we don't care, hash functions usually don't error in ways we
care about), and rollbacks of non-writing transactions to a
database. After studying the boltdb docs, I concluded that
the recommended solution is to use the `.View(...)` function
instead of directly controlling the transaction, so I switched
the functions to do that.
