[WIP] Store Sync (#1365)

* Initial cache implementation

* Write queue implementation

* Accidentally started writing the storage sync service

sync/store/manager.go

@@ -0,0 +1,89 @@
+package store
+
+import (
+	"time"
+
+	"github.com/micro/go-micro/v2/store"
+	"github.com/pkg/errors"
+)
+
+type operation struct {
+	operation action
+	record    *store.Record
+	deadline  time.Time
+	retries   int
+	maxiumum  int
+}
+
+// action represents the type of a queued operation
+type action int
+
+const (
+	readOp action = iota + 1
+	writeOp
+	deleteOp
+	listOp
+)
+
+func (c *cache) cacheManager() {
+	tickerAggregator := make(chan struct{ index int })
+	for i, ticker := range c.pendingWriteTickers {
+		go func(index int, c chan struct{ index int }, t *time.Ticker) {
+			for range t.C {
+				c <- struct{ index int }{index: index}
+			}
+		}(i, tickerAggregator, ticker)
+	}
+	for {
+		select {
+		case i := <-tickerAggregator:
+			println(i.index, "ticked")
+			c.processQueue(i.index)
+		}
+	}
+}
+
+func (c *cache) processQueue(index int) {
+	c.Lock()
+	defer c.Unlock()
+	q := c.pendingWrites[index]
+	for i := 0; i < q.Len(); i++ {
+		r, ok := q.PopFront()
+		if !ok {
+			panic(errors.Errorf("retrieved an invalid value from the L%d cache queue", index+1))
+		}
+		ir, ok := r.(*internalRecord)
+		if !ok {
+			panic(errors.Errorf("retrieved a non-internal record from the L%d cache queue", index+1))
+		}
+		if !ir.expiresAt.IsZero() && time.Now().After(ir.expiresAt) {
+			continue
+		}
+		nr := &store.Record{
+			Key: ir.key,
+		}
+		nr.Value = make([]byte, len(ir.value))
+		copy(nr.Value, ir.value)
+		if !ir.expiresAt.IsZero() {
+			nr.Expiry = time.Until(ir.expiresAt)
+		}
+		// Todo = internal queue also has to hold the corresponding store.WriteOptions
+		if err := c.cOptions.Stores[index+1].Write(nr); err != nil {
+			// some error, so queue for retry and bail
+			q.PushBack(ir)
+			return
+		}
+	}
+}
+
+func intpow(x, y int64) int64 {
+	result := int64(1)
+	for 0 != y {
+		if 0 != (y & 1) {
+			result *= x
+		}
+		y >>= 1
+		x *= x
+	}
+	return result
+}