db: Simplify exists check, fix IsKeyInDomain signature, Iterator Close

+ *FSDB.HasKey now uses common.FileExists to test for file existence
+ IsKeyInDomain takes key as a []byte slice instead of as a string
to avoid extraneous []byte<-->string conversions for start and end
+ Iterator.Close() instead of Iterator.Release()
+ withDB helper to encapsulate DB creation, deferred cleanups
so that for loops can use opened DBs and discard them ASAP

Addressing accepted changes from review with @jaekwon

db/types.go

@@ -68,7 +68,7 @@ func EndingKey() []byte {
 	Usage:
 
 	var itr Iterator = ...
-	defer itr.Release()
+	defer itr.Close()
 
 	for ; itr.Valid(); itr.Next() {
 		k, v := itr.Key(); itr.Value()
@@ -108,7 +108,7 @@ type Iterator interface {
 	Value() []byte
 
 	// Release deallocates the given Iterator.
-	Release()
+	Close()
 }
 
 // For testing convenience.