Introduce state/toolstorage

state/toolstorage provides an interface, Storage,
for storing tools tarballs and metadata in state.
The tarballs are stored in GridFS, while the
metadata are stored in a Mongo collection.

State grows a single new method, ToolsStorage,
which constructs a toolstorage.StorageCloser.
The returned object will store tools metadata in
the "juju" database's "toolsmetadata" collection,
and the tarballs in the "blobstore" database.
In that case can we remove the old tools blob once the new one is written?