trylock: Use sync.Mutex in go1.18+

Go 1.18 supplements Mutex with a TryLock method.
Use this directly instead of our dumb homebrew
trylock mutex when Go 1.18 is in use.

CI with go 1.18-beta1

Author: chappjc

.github/workflows/build.yml

@@ -7,14 +7,15 @@ jobs:
     strategy:
       matrix:
         node-version: [14.x, 16.x]
-        go: [1.16, 1.17]
+        go: [1.17, 1.18.0-beta1]
     steps:
     - uses: actions/checkout@v2
 
     - name: Set up Go
       uses: actions/setup-go@v2
       with:
         go-version: ${{ matrix.go }}
+        stable: '!contains(${{ matrix.go }}, "beta") && !contains(${{ matrix.go }}, "rc")'
 
     - name: Go Build
       env:

trylock/trylock.go

@@ -1,6 +1,9 @@
 // Copyright (c) 2021, The Decred developers
 // See LICENSE for details.
 
+//go:build !go1.18
+// +build !go1.18
+
 package trylock
 
 import (

trylock/trylock_go1.18.go

@@ -0,0 +1,13 @@
+// Copyright (c) 2021, The Decred developers
+// See LICENSE for details.
+
+//go:build go1.18
+// +build go1.18
+
+package trylock
+
+import "sync"
+
+// Mutex is just an alias for sync.Mutex in Go 1.18 onward since TryLock() was
+// added to the standard library.
+type Mutex = sync.Mutex