Light clients require full nodes to serve additional data so that they
can stay in sync with the network. This patch adds a new launch option
`--serve-light-client-data` to enable collection of light client data.
`--import-light-client-data` configures the classes of data to import.
This can be set to `none`, `only-new`, `full`, or `on-demand`.
Note that data is only locally collected, a separate patch is needed to
actually make it availble over the network. Likewise, data is only kept
in memory; it is not persisted at this time.
Etan Kissling