Add system activity tracking and event store (#636)

This PR adds system activity tracking. 
The management service records events like 
add/remove peer,  group, rule, route, etc.

The activity events are stored in the SQLite event store
and can be queried by the HTTP API.

management/server/activity/store.go

@@ -0,0 +1,54 @@
+package activity
+
+import "sync"
+
+// Store provides an interface to store or stream events.
+type Store interface {
+	// Save an event in the store
+	Save(event *Event) (*Event, error)
+	// Get returns "limit" number of events from the "offset" index ordered descending or ascending by a timestamp
+	Get(accountID string, offset, limit int, descending bool) ([]*Event, error)
+	// Close the sink flushing events if necessary
+	Close() error
+}
+
+// InMemoryEventStore implements the Store interface storing data in-memory
+type InMemoryEventStore struct {
+	mu     sync.Mutex
+	nextID uint64
+	events []*Event
+}
+
+// Save sets the Event.ID to 1
+func (store *InMemoryEventStore) Save(event *Event) (*Event, error) {
+	store.mu.Lock()
+	defer store.mu.Unlock()
+	if store.events == nil {
+		store.events = make([]*Event, 0)
+	}
+	event.ID = store.nextID
+	store.nextID++
+	store.events = append(store.events, event)
+	return event, nil
+}
+
+// Get returns a list of ALL events that belong to the given accountID without taking offset, limit and order into consideration
+func (store *InMemoryEventStore) Get(accountID string, offset, limit int, descending bool) ([]*Event, error) {
+	store.mu.Lock()
+	defer store.mu.Unlock()
+	events := make([]*Event, 0)
+	for _, event := range store.events {
+		if event.AccountID == accountID {
+			events = append(events, event)
+		}
+	}
+	return events, nil
+}
+
+// Close cleans up the event list
+func (store *InMemoryEventStore) Close() error {
+	store.mu.Lock()
+	defer store.mu.Unlock()
+	store.events = make([]*Event, 0)
+	return nil
+}