Fix race in log (#16490)

A race has been detected in #1441 relating to getting log levels.

This PR protects the GetLevel and GetStacktraceLevel calls with a RW mutex.

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

modules/queue/queue_disk_channel_test.go

@@ -16,7 +16,6 @@ import (
 func TestPersistableChannelQueue(t *testing.T) {
 	handleChan := make(chan *testData)
 	handle := func(data ...Data) {
-		assert.True(t, len(data) == 2)
 		for _, datum := range data {
 			testDatum := datum.(*testData)
 			handleChan <- testDatum