TUN-8861: Rename Session Limiter to Flow Limiter

## Summary
Session is the concept used for UDP flows. Therefore, to make
the session limiter ambiguous for both TCP and UDP, this commit
renames it to flow limiter.

Closes TUN-8861

flow/limiter_test.go

@@ -0,0 +1,119 @@
+package flow_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/cloudflare/cloudflared/flow"
+)
+
+func TestFlowLimiter_Unlimited(t *testing.T) {
+	unlimitedLimiter := flow.NewLimiter(0)
+
+	for i := 0; i < 1000; i++ {
+		err := unlimitedLimiter.Acquire("test")
+		require.NoError(t, err)
+	}
+}
+
+func TestFlowLimiter_Limited(t *testing.T) {
+	maxFlows := uint64(5)
+	limiter := flow.NewLimiter(maxFlows)
+
+	for i := uint64(0); i < maxFlows; i++ {
+		err := limiter.Acquire("test")
+		require.NoError(t, err)
+	}
+
+	err := limiter.Acquire("should fail")
+	require.ErrorIs(t, err, flow.ErrTooManyActiveFlows)
+}
+
+func TestFlowLimiter_AcquireAndReleaseFlow(t *testing.T) {
+	maxFlows := uint64(5)
+	limiter := flow.NewLimiter(maxFlows)
+
+	// Acquire the maximum number of flows
+	for i := uint64(0); i < maxFlows; i++ {
+		err := limiter.Acquire("test")
+		require.NoError(t, err)
+	}
+
+	// Validate acquire 1 more flows fails
+	err := limiter.Acquire("should fail")
+	require.ErrorIs(t, err, flow.ErrTooManyActiveFlows)
+
+	// Release the maximum number of flows
+	for i := uint64(0); i < maxFlows; i++ {
+		limiter.Release()
+	}
+
+	// Validate acquire 1 more flows works
+	err = limiter.Acquire("shouldn't fail")
+	require.NoError(t, err)
+
+	// Release a 10x the number of max flows
+	for i := uint64(0); i < 10*maxFlows; i++ {
+		limiter.Release()
+	}
+
+	// Validate it still can only acquire a value = number max flows.
+	for i := uint64(0); i < maxFlows; i++ {
+		err := limiter.Acquire("test")
+		require.NoError(t, err)
+	}
+	err = limiter.Acquire("should fail")
+	require.ErrorIs(t, err, flow.ErrTooManyActiveFlows)
+}
+
+func TestFlowLimiter_SetLimit(t *testing.T) {
+	maxFlows := uint64(5)
+	limiter := flow.NewLimiter(maxFlows)
+
+	// Acquire the maximum number of flows
+	for i := uint64(0); i < maxFlows; i++ {
+		err := limiter.Acquire("test")
+		require.NoError(t, err)
+	}
+
+	// Validate acquire 1 more flows fails
+	err := limiter.Acquire("should fail")
+	require.ErrorIs(t, err, flow.ErrTooManyActiveFlows)
+
+	// Set the flow limiter to support one more request
+	limiter.SetLimit(maxFlows + 1)
+
+	// Validate acquire 1 more flows now works
+	err = limiter.Acquire("shouldn't fail")
+	require.NoError(t, err)
+
+	// Validate acquire 1 more flows doesn't work because we already reached the limit
+	err = limiter.Acquire("should fail")
+	require.ErrorIs(t, err, flow.ErrTooManyActiveFlows)
+
+	// Release all flows
+	for i := uint64(0); i < maxFlows+1; i++ {
+		limiter.Release()
+	}
+
+	// Validate 1 flow works again
+	err = limiter.Acquire("shouldn't fail")
+	require.NoError(t, err)
+
+	// Set the flow limit to 1
+	limiter.SetLimit(1)
+
+	// Validate acquire 1 more flows doesn't work
+	err = limiter.Acquire("should fail")
+	require.ErrorIs(t, err, flow.ErrTooManyActiveFlows)
+
+	// Set the flow limit to unlimited
+	limiter.SetLimit(0)
+
+	// Validate it can acquire a lot of flows because it is now unlimited.
+	for i := uint64(0); i < 10*maxFlows; i++ {
+		err := limiter.Acquire("shouldn't fail")
+		require.NoError(t, err)
+	}
+}