feat(transcode): add cache pruning and config options

* Added config option to set size of transcode cache and cadence to enforce that sizing via ejection.

* Added cache eject to contrib/config.

* Added error return for CacheEject(). Changed to use WalkDir() instead of Walk().

* Lint fix.

* Added universal lock for cache eject.

* Removed accidentally committed binary.

transcode/transcode_test.go

@@ -113,7 +113,7 @@ func TestCachingParallelism(t *testing.T) {
 		callback:   func() { realTranscodeCount.Add(1) },
 	}
 
-	cacheTranscoder := transcode.NewCachingTranscoder(transcoder, t.TempDir())
+	cacheTranscoder := transcode.NewCachingTranscoder(transcoder, t.TempDir(), 1024)
 
 	var wg sync.WaitGroup
 	for i := 0; i < 5; i++ {