lite2: make maxClockDrift an option (#4616)

Closes #4607

docs/architecture/adr-046-light-client-implementation.md

@@ -4,6 +4,7 @@
 * 13-02-2020: Initial draft
 * 26-02-2020: Cross-checking the first header
 * 28-02-2020: Bisection algorithm details
+* 31-03-2020: Verify signature got changed
 
 ## Context
 
@@ -100,6 +101,10 @@ type Store interface {
 	FirstSignedHeaderHeight() (int64, error)
 
 	SignedHeaderAfter(height int64) (*types.SignedHeader, error)
+
+	Prune(size uint16) error
+
+	Size() uint16
 }
 ```
 
@@ -110,12 +115,13 @@ database, used in Tendermint). In the future, remote adapters are possible
 ```go
 func Verify(
 	chainID string,
-	h1 *types.SignedHeader,
-	h1NextVals *types.ValidatorSet,
-	h2 *types.SignedHeader,
-	h2Vals *types.ValidatorSet,
+	trustedHeader *types.SignedHeader, // height=X
+	trustedVals *types.ValidatorSet, // height=X or height=X+1
+	untrustedHeader *types.SignedHeader, // height=Y
+	untrustedVals *types.ValidatorSet, // height=Y
 	trustingPeriod time.Duration,
 	now time.Time,
+	maxClockDrift time.Duration,
 	trustLevel tmmath.Fraction) error {
 ```
 
@@ -124,6 +130,9 @@ cases of adjacent and non-adjacent headers. In the former case, it compares the
 hashes directly (2/3+ signed transition). Otherwise, it verifies 1/3+
 (`trustLevel`) of trusted validators are still present in new validators.
 
+While `Verify` function is certainly handy, `VerifyAdjacent` and
+`VerifyNonAdjacent` should be used most often to avoid logic errors.
+
 ### Bisection algorithm details
 
 Non-recursive bisection algorithm was implemented despite the spec containing