Add API Token Cache (#16547)

One of the issues holding back performance of the API is the problem of hashing.
Whilst banning BASIC authentication with passwords will help, the API Token scheme
still requires a PBKDF2 hash - which means that heavy API use (using Tokens) can
still cause enormous numbers of hash computations.

A slight solution to this whilst we consider moving to using JWT based tokens and/or
a session orientated solution is to simply cache the successful tokens. This has some
security issues but this should be balanced by the security issues of load from
hashing.

Related #14668

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

models/models.go

@@ -17,6 +17,7 @@ import (
 
 	// Needed for the MySQL driver
 	_ "github.com/go-sql-driver/mysql"
+	lru "github.com/hashicorp/golang-lru"
 	"xorm.io/xorm"
 	"xorm.io/xorm/names"
 	"xorm.io/xorm/schemas"
@@ -234,6 +235,15 @@ func NewEngine(ctx context.Context, migrateFunc func(*xorm.Engine) error) (err e
 		return fmt.Errorf("sync database struct error: %v", err)
 	}
 
+	if setting.SuccessfulTokensCacheSize > 0 {
+		successfulAccessTokenCache, err = lru.New(setting.SuccessfulTokensCacheSize)
+		if err != nil {
+			return fmt.Errorf("unable to allocate AccessToken cache: %v", err)
+		}
+	} else {
+		successfulAccessTokenCache = nil
+	}
+
 	return nil
 }