Fix compactions sometimes getting stuck

I ran into an issue where the cache snapshotting seemed to stop
completely causing the cache to fill up and never recover.  I believe
this is due to the the Timer being reused incorrectly.  Instead,
use a Ticker that will fire more regularly and not require the resetting
logic (which was wrong).

tsdb/engine/tsm1/engine.go

@@ -949,7 +949,7 @@ func (e *Engine) writeSnapshotAndCommit(closedFiles []string, snapshot *Cache) (
 
 // compactCache continually checks if the WAL cache should be written to disk.
 func (e *Engine) compactCache(quit <-chan struct{}) {
-	t := time.NewTimer(time.Second)
+	t := time.NewTicker(time.Second)
 	defer t.Stop()
 	for {
 		select {
@@ -971,7 +971,6 @@ func (e *Engine) compactCache(quit <-chan struct{}) {
 				atomic.AddInt64(&e.stats.CacheCompactionDuration, time.Since(start).Nanoseconds())
 			}
 		}
-		t.Reset(time.Second)
 	}
 }
 
@@ -989,7 +988,7 @@ func (e *Engine) ShouldCompactCache(lastWriteTime time.Time) bool {
 }
 
 func (e *Engine) compactTSMLevel(fast bool, level int, quit <-chan struct{}) {
-	t := time.NewTimer(time.Second)
+	t := time.NewTicker(time.Second)
 	defer t.Stop()
 
 	for {
@@ -1003,12 +1002,11 @@ func (e *Engine) compactTSMLevel(fast bool, level int, quit <-chan struct{}) {
 				s.Apply()
 			}
 		}
-		t.Reset(time.Second)
 	}
 }
 
 func (e *Engine) compactTSMFull(quit <-chan struct{}) {
-	t := time.NewTimer(time.Second)
+	t := time.NewTicker(time.Second)
 	defer t.Stop()
 
 	for {
@@ -1023,7 +1021,6 @@ func (e *Engine) compactTSMFull(quit <-chan struct{}) {
 			}
 
 		}
-		t.Reset(time.Second)
 	}
 }