Create time Package for Shared/timeutil, mclock and slotutil (#9594)

* add time pkg

* Go fmt

time/mclock/BUILD.bazel

@@ -0,0 +1,9 @@
+load("@prysm//tools/go:def.bzl", "go_library")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["mclock.go"],
+    importpath = "github.com/prysmaticlabs/prysm/time/mclock",
+    visibility = ["//visibility:public"],
+    deps = ["@com_github_aristanetworks_goarista//monotime:go_default_library"],
+)

time/mclock/mclock.go

@@ -0,0 +1,63 @@
+// Copyright 2016 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+// Package mclockutil is a wrapper for a monotonic clock source.
+package mclock
+
+import (
+	"time"
+
+	"github.com/aristanetworks/goarista/monotime"
+)
+
+// AbsTime represents absolute monotonic time.
+type AbsTime time.Duration
+
+// Now returns the current absolute monotonic time.
+func Now() AbsTime {
+	return AbsTime(monotime.Now())
+}
+
+// Add returns t + d.
+func (t AbsTime) Add(d time.Duration) AbsTime {
+	return t + AbsTime(d)
+}
+
+// Clock interface makes it possible to replace the monotonic system clock with
+// a simulated clock.
+type Clock interface {
+	Now() AbsTime
+	Sleep(time.Duration)
+	After(time.Duration) <-chan time.Time
+}
+
+// System implements Clock using the system clock.
+type System struct{}
+
+// Now implements Clock.
+func (System) Now() AbsTime {
+	return AbsTime(monotime.Now())
+}
+
+// Sleep implements Clock.
+func (System) Sleep(d time.Duration) {
+	time.Sleep(d)
+}
+
+// After implements Clock.
+func (System) After(d time.Duration) <-chan time.Time {
+	return time.After(d)
+}