Add WriteTxn interface and convert more functions to ReadTxn

We add a WriteTxn interface for use in updating the usage memdb table,
with the forward-looking prospect of incrementally converting other
functions to accept interfaces.

As well, we use the ReadTxn in new usage code, and as a side effect
convert a couple of existing functions to use that interface as well.

agent/consul/state/memdb.go

@@ -15,6 +15,13 @@ type ReadTxn interface {
 	Abort()
 }
 
+// WriteTxn is implemented by memdb.Txn to perform write operations.
+type WriteTxn interface {
+	ReadTxn
+	Insert(table string, obj interface{}) error
+	Commit() error
+}
+
 // Changes wraps a memdb.Changes to include the index at which these changes
 // were made.
 type Changes struct {

agent/consul/state/usage.go

@@ -37,7 +37,7 @@ type UsageEntry struct {
 
 // updateUsage takes a set of memdb changes and computes a delta for specific
 // usage metrics that we track.
-func updateUsage(tx *txn, changes Changes) error {
+func updateUsage(tx WriteTxn, changes Changes) error {
 	usageDeltas := make(map[string]int)
 	for _, change := range changes.Changes {
 		var delta int
@@ -140,7 +140,7 @@ func (s *Store) ServiceUsage() (uint64, ServiceUsage, error) {
 		return 0, ServiceUsage{}, fmt.Errorf("failed services lookup: %s", err)
 	}
 
-	results, err := s.compileServiceUsage(tx, usage.Count)
+	results, err := compileServiceUsage(tx, usage.Count)
 	if err != nil {
 		return 0, ServiceUsage{}, fmt.Errorf("failed services lookup: %s", err)
 	}
@@ -148,7 +148,7 @@ func (s *Store) ServiceUsage() (uint64, ServiceUsage, error) {
 	return usage.Index, results, nil
 }
 
-func firstUsageEntry(tx *txn, id string) (*UsageEntry, error) {
+func firstUsageEntry(tx ReadTxn, id string) (*UsageEntry, error) {
 	usage, err := tx.First("usage", "id", id)
 	if err != nil {
 		return nil, err

agent/consul/state/usage_oss.go

@@ -12,7 +12,7 @@ type EnterpriseServiceUsage struct{}
 
 func addEnterpriseUsage(map[string]int, memdb.Change) {}
 
-func (s *Store) compileServiceUsage(tx *txn, totalInstances int) (ServiceUsage, error) {
+func compileServiceUsage(tx ReadTxn, totalInstances int) (ServiceUsage, error) {
 	var totalServices int
 	results, err := tx.Get(
 		"index",