* 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)
* follow up to removing some consensus params Refs #2382
* change args type to int64 in state#makeParams
* make valsCount and evidenceCount ints again
* MaxEvidenceBytesPerBlock: include magic number in godoc
* [spec] creating a proposal
* test state#TxFilter
* panic if MaxDataBytes is less than 0
* fixes after review
* use amino#UvarintSize to calculate overhead
0c74291f3b/encoder.go (L85-L90)
* avoid cyclic imports
* you can do better Go, come on
* remove testdouble package
* use increment and decrement operators.
* remove unnecessary else branches.
* fix package comment with leading space.
* fix receiver names.
* fix error strings.
* remove omittable code.
* remove redundant return statement.
* Revert changes (code is generated.)
* use cfg as receiver name for all config-related types.
* use lsi as the receiver name for the LastSignedInfo type.
