genesis: add support for arbitrary initial height (#5191)

Adds a genesis parameter `initial_height` which specifies the initial block height, as well as ABCI `RequestInitChain.InitialHeight` to pass it to the ABCI application, and `State.InitialHeight` to keep track of the initial height throughout the code. Fixes #2543, based on [RFC-002](https://github.com/tendermint/spec/pull/119). Spec changes in https://github.com/tendermint/spec/pull/135.

evidence/pool_test.go

@@ -491,6 +491,7 @@ func initializeStateFromValidatorSet(valSet *types.ValidatorSet, height int64) d
 	stateDB := dbm.NewMemDB()
 	state := sm.State{
 		ChainID:                     evidenceChainID,
+		InitialHeight:               1,
 		LastBlockHeight:             height,
 		LastBlockTime:               tmtime.Now(),
 		Validators:                  valSet,