breaking! Remove internal mutex boxing of connections

[knutwalker]

Connections are no longer always wrapped in an Arc<Mutex<>>.

This actually introduces a number of breaking changes:

• Graph::execute returns a DetachedRowStream instead of a RowStream (name pending)
  The API is identical, but it's a different type
• Txn::execute continues to return a RowStream, but its API changed:
  • RowStream::next required a TransactionHandle, which can be obtained by calling Txn::handle
  • This method is defined as fn handle(&mut self), which means that the synchronization of concurrent query streams within a single transactions must now be handled by the user
  • The same is true for all other methods consuming the RowStream
• Txn methods now require &mut self instead of &self