core: Add _STRING variants of GVariant object formats

For the static deltas work, we're going to embed a commit in the delta
superblock, so we need a format string without the G_VARIANT_TYPE().
otutil: Correctly add ref to bytes when creating GVariant

This one went undiscovered for a while because it turned out we
weren't using it...
libostree: Add _finish() API to async progress

Since OstreeAsyncProgress queues to the mainloop, we might "lose" the
last message.  Give callers a way to force a flush.
deltas: Link to liblzma, add internal API to use it

For future static deltas work, we'll be linking to liblzma.  Since
it's fairly widespread, let's just make it a hard dependency.
pull: Stay in mainloop if we're synchronously fetching URI

The static deltas work will be doing some synchronous fetching even
after refs are downloaded.
pull: Correctly handle error state when fetching optional data

For the static deltas work, we're using the already-extant internal
API to perform a HTTP fetch for optional data - static deltas are
optional.

Except that we didn't correctly unset the error if we were doing an
optional fetch and the data wasn't found.