lite: Add synchronization in lite verify (#2396)

* Implement issues 2386: add synchronization in lite verify and change all Certify to Verify

* Replace make(chan struct{}, 0) with make(chan struct{})

* Parameterize memroy cache size and add concurrent test

* Refactor import order

lite/types.go

@@ -8,6 +8,6 @@ import (
 // Verifier must know the current or recent set of validitors by some other
 // means.
 type Verifier interface {
-	Certify(sheader types.SignedHeader) error
+	Verify(sheader types.SignedHeader) error
 	ChainID() string
 }