all, state: unexpose GenesisDoc, ChainID fields make them accessor methods

Fixes #671

Unexpose GenesisDoc and ChainID fields to avoid them being
serialized to the DB on every block write/state.Save()

A GenesisDoc can now be alternatively written to the state's
database, by serializing its JSON as a value of key "genesis-doc".

There are now accessors and a setter for these attributes:
- state.GenesisDoc() (*types.GenesisDoc, error)
- state.ChainID() (string, error)
- state.SetGenesisDoc(*types.GenesisDoc)

This is a breaking change since it changes how the state's
serialization and requires that if loading the GenesisDoc entirely
from the database, you'll need to set its value in the database
as the GenesisDoc's JSON marshaled bytes.

blockchain/reactor.go

@@ -242,8 +242,14 @@ FOR_LOOP:
 				// NOTE: we can probably make this more efficient, but note that calling
 				// first.Hash() doesn't verify the tx contents, so MakePartSet() is
 				// currently necessary.
-				err := bcR.state.Validators.VerifyCommit(
-					bcR.state.ChainID, types.BlockID{first.Hash(), firstPartsHeader}, first.Height, second.LastCommit)
+				chainID, err := bcR.state.ChainID()
+				if err != nil {
+					bcR.Logger.Info("error in retrieving chainID", "err", err)
+					bcR.pool.RedoRequest(first.Height)
+					break SYNC_LOOP
+				}
+				err = bcR.state.Validators.VerifyCommit(
+					chainID, types.BlockID{first.Hash(), firstPartsHeader}, first.Height, second.LastCommit)
 				if err != nil {
 					bcR.Logger.Error("Error in validation", "err", err)
 					bcR.pool.RedoRequest(first.Height)