slave node design

[syntrust]

Tracking QuarkChain/goshard#17

[qzhodl]

Since the GenesisMeta record (prev-root-block hash, xshard cursor, full shard id) is a temporary scaffold that exists only because geth's stock header has no home for these fields yet, the implementation should mark it as such explicitly — not just note it in this design doc.

Concretely, when implementing:

• Tag the GenesisMeta struct, its rawdb accessors, and the Reconcile() path with a grep-able marker, e.g. // TODO: temporary — remove once QKC block format lands; these fields move into the genesis block's header/meta and Reconcile() should compare the genesis block itself.
• Make the marker say plainly that once the block issue merges, this code is re-implemented, not patched: the fields are read from the genesis block's header/meta, and Reconcile() switches to the geth-native genesis-hash check (SetupGenesisBlock-style). The GenesisMeta record is then deleted, not migrated — at that stage the db only holds the genesis block, so --clean re-bootstrap suffices and no migration code is needed.