Add a sync_fs method that writes dirty items into level 0 item blocks.
Add chunk allocator code to allocate new item blocks in free chunks. As
the allocator bitmap is modified it adds bitmap entries to the ring.
As new item blocks are allocated we create manifest entries that
describe their block location and keys. The entry is added to the
in-memory manifest and to entries in the ring.
This isn't complete and there's still bugs but this is enough to start
building on.
Signed-off-by: Zach Brown <zab@versity.com>