evidence: Add time to evidence params (#69)

* evidence: Add time to evidence params

- this pr is grouped together with https://github.com/tendermint/tendermint/pull/4254, once that PR is merged then this one can be as well.

Signed-off-by: Marko Baricevic <marbar3778@yahoo.com>

* remove note

Signed-off-by: Marko Baricevic <marbar3778@yahoo.com>

* Apply suggestions from code review

Co-Authored-By: Anton Kaliaev <anton.kalyaev@gmail.com>

Co-authored-by: Anton Kaliaev <anton.kalyaev@gmail.com>

spec/abci/abci.md

@@ -501,11 +501,12 @@ Commit are included in the header of the next block.
 ### EvidenceParams
 
 - **Fields**:
-  - `MaxAge (int64)`: Max age of evidence, in blocks. Evidence older than this
-    is considered stale and ignored.
+  - `MaxAgeNumBlocks (int64)`: Max age of evidence, in blocks.
+  - `MaxAgeDuration (time.Duration)`: Max age of evidence, in time.
+    Evidence older than this is considered stale and ignored.
+    
     - This should correspond with an app's "unbonding period" or other
       similar mechanism for handling Nothing-At-Stake attacks.
-    - NOTE: this should change to time (instead of blocks)!
 
 ### ValidatorParams
 

spec/blockchain/state.md

@@ -110,7 +110,8 @@ type BlockParams struct {
 }
 
 type EvidenceParams struct {
-	MaxAge int64
+    MaxAgeNumBlocks    int64
+    MaxAgeDuration  time.Duration
 }
 
 type ValidatorParams struct {
@@ -135,7 +136,8 @@ The minimal time between consecutive blocks is controlled by the
 For evidence in a block to be valid, it must satisfy:
 
 ```
-block.Header.Height - evidence.Height < ConsensusParams.Evidence.MaxAge
+block.Header.Height - evidence.Height < ConsensusParams.Evidence.MaxAgeNumBlocks
+block.Header.Time - evidence.Time < ConsensusParams.Evidence.MaxAgeDuration
 ```
 
 #### Validator