state: require block.Time of the fist block to be genesis time (#2594)

* require block.Time of the fist block to be genesis time

Refs #2587:

```
We only start validating block.Time when Height > 1, because there is no
commit to compute the median timestamp from for the first block. This
means a faulty proposer could make the first block with whatever time
they want.

Instead, we should require the timestamp of block 1 to match the genesis
time.

I discovered this while refactoring the ValidateBlock tests to be
table-driven while working on tests for #2560.
```

* do not accept blocks with negative height

* update changelog and spec

* nanos precision for test genesis time

* Fix failing test (#2607)

config/toml.go

@@ -342,7 +342,7 @@ func ResetTestRoot(testName string) *Config {
 }
 
 var testGenesis = `{
-  "genesis_time": "0001-01-01T00:00:00.000Z",
+  "genesis_time": "2017-10-10T08:20:13.695936996Z",
   "chain_id": "tendermint_test",
   "validators": [
     {