server: replace mlockall with Mlock in --experimental-memory-mlock

Implementation of `--experimental-memory-mlock` backed by `mlockall` syscall is replaced by `Mlock` flag (backed by mlock syscall) of bboltDB.

server/mvcc/backend/backend.go

@@ -122,6 +122,8 @@ type BackendConfig struct {
 	Logger *zap.Logger
 	// UnsafeNoFsync disables all uses of fsync.
 	UnsafeNoFsync bool `json:"unsafe-no-fsync"`
+	// Mlock prevents backend database file to be swapped
+	Mlock bool
 }
 
 func DefaultBackendConfig() BackendConfig {
@@ -155,6 +157,7 @@ func newBackend(bcfg BackendConfig) *backend {
 	bopts.FreelistType = bcfg.BackendFreelistType
 	bopts.NoSync = bcfg.UnsafeNoFsync
 	bopts.NoGrowSync = bcfg.UnsafeNoFsync
+	bopts.Mlock = bcfg.Mlock
 
 	db, err := bolt.Open(bcfg.Path, 0600, bopts)
 	if err != nil {
@@ -381,6 +384,8 @@ func (b *backend) defrag() error {
 	options.OpenFile = func(path string, i int, mode os.FileMode) (file *os.File, err error) {
 		return temp, nil
 	}
+	// Don't load tmp db into memory regardless of opening options
+	options.Mlock = false
 	tdbp := temp.Name()
 	tmpdb, err := bolt.Open(tdbp, 0600, &options)
 	if err != nil {