This change introduces garbage collection for the light client. Historical
chain data is deleted periodically. If you want to disable the GC, use
the --light.nopruning flag.
* les: move the checkpoint oracle into its own package
It's first step of refactor LES package. LES package
basically can be divided into LES client and LES server.
However both sides will use checkpoint package for
status retrieval and verification. So this PR moves
checkpoint oracle into a separate package
* les: address comments