authorize: track session and service account access date (#3220)

* session: add accessed at date

* authorize: track session and service account access times

* Revert "databroker: add support for field masks on Put (#3210)"

This reverts commit 2dc778035d.

* add test

* fix data race in test

* add deadline for update

* track dropped accesses

internal/sets/sets.go

@@ -0,0 +1,2 @@
+// Package sets contains set data structures.
+package sets

internal/sets/size_limited.go

@@ -0,0 +1,36 @@
+package sets
+
+// A SizeLimitedStringSet is a StringSet which is limited to a given size. Once
+// the capacity is reached an element will be removed at random.
+type SizeLimitedStringSet struct {
+	m        map[string]struct{}
+	capacity int
+}
+
+// NewSizeLimitedStringSet create a new SizeLimitedStringSet.
+func NewSizeLimitedStringSet(capacity int) *SizeLimitedStringSet {
+	return &SizeLimitedStringSet{
+		m:        make(map[string]struct{}),
+		capacity: capacity,
+	}
+}
+
+// Add adds an element to the set.
+func (s *SizeLimitedStringSet) Add(element string) {
+	s.m[element] = struct{}{}
+	for len(s.m) > s.capacity {
+		for k := range s.m {
+			delete(s.m, k)
+			break
+		}
+	}
+}
+
+// ForEach iterates over all the elements in the set.
+func (s *SizeLimitedStringSet) ForEach(callback func(element string) bool) {
+	for k := range s.m {
+		if !callback(k) {
+			return
+		}
+	}
+}