What these generate is not really suitable to be used as a filename,
which is why keyFile and fileKey further escape it. These are just
serializing Keys.
Also removed a quickcheck test that was very unlikely to test anything
useful, since it relied on random chance creating something that looks
like a serialized key. The other test is sufficient for testing what
that was intended to test anyway.
Also, rename lockContent to lockContentExclusive
inAnnexSafe should perhaps be eliminated, and instead use
`lockContentShared inAnnex`. However, I'm waiting on that, as there are
only 2 call sites for inAnnexSafe and it's fiddly.
This fixed one bug where it needed to be and wasn't (in Assistant.Unused).
And also found one place where lockContent was used unnecessarily (by
drop --from remote).
A few other places like uninit probably don't really need to lockContent,
but it doesn't hurt to do call it anyway.
This commit was sponsored by David Wagner.
