testnet: add consensus_params to testnet config generation (#3781)

Also, document time_iota_ms.

Closes #3723

Commits:

* config: Add ConsensusParams when generating testnet config (#3723)

* docs: Add explanation of time_iota_ms (#3723)

* Update changelog_pending (#3723)

docs/tendermint-core/using-tendermint.md

@@ -43,6 +43,11 @@ definition](https://github.com/tendermint/tendermint/blob/master/types/genesis.g
 - `chain_id`: ID of the blockchain. This must be unique for
   every blockchain. If your testnet blockchains do not have unique
   chain IDs, you will have a bad time. The ChainID must be less than 50 symbols.
+- `consensus_params`
+  - `block`
+    - `time_iota_ms`: Minimum time increment between consecutive blocks (in
+      milliseconds). If the block header timestamp is ahead of the system clock,
+      decrease this value.
 - `validators`: List of initial validators. Note this may be overridden entirely by the
   application, and may be left empty to make explicit that the
   application will initialize the validator set with ResponseInitChain.
@@ -63,9 +68,10 @@ definition](https://github.com/tendermint/tendermint/blob/master/types/genesis.g
   "genesis_time": "2018-11-13T18:11:50.277637Z",
   "chain_id": "test-chain-s4ui7D",
   "consensus_params": {
-    "block_size": {
+    "block": {
       "max_bytes": "22020096",
-      "max_gas": "-1"
+      "max_gas": "-1",
+      "time_iota_ms": "1000"
     },
     "evidence": {
       "max_age": "100000"