Move most code from the repo root into subdirectories

The intention is to keep to repo root clean while the list of packages
is slowly growing.

Additionally, a bunch of small (~30 LoC) files in the repo root is
merged into a single maddy.go file, for the same reason.

Most of the internal code is moved into the internal/ directory. Go
toolchain will make it impossible to import these packages from external
applications.

Some packages are renamed and moved into the pkg/ directory in the root.
According to https://github.com/golang-standards/project-layout this is
the de-facto standard to place "library code that's ok to use by
external applications" in.

To clearly define the purpose of top-level directories, README.md files
are added to each.

internal/storage/sql/bench_test.go

@@ -0,0 +1,86 @@
+package sql
+
+import (
+	"flag"
+	"math/rand"
+	"strconv"
+	"testing"
+	"time"
+
+	imapsql "github.com/foxcpp/go-imap-sql"
+	"github.com/foxcpp/maddy/internal/testutils"
+)
+
+var (
+	testDB      string
+	testDSN     string
+	testFsstore string
+)
+
+func init() {
+	flag.StringVar(&testDB, "sql.testdb", "", "Database to use for storage/sql benchmarks")
+	flag.StringVar(&testDSN, "sql.testdsn", "", "DSN to use for storage/sql benchmarks")
+	flag.StringVar(&testFsstore, "sql.testfsstore", "", "fsstore location to use for storage/sql benchmarks")
+}
+
+func createTestDB(tb testing.TB, compAlgo string) *Storage {
+	if testDB == "" || testDSN == "" || testFsstore == "" {
+		tb.Skip("-sql.testdb, -sql.testdsn and -sql.testfsstore should be specified to run this benchmark")
+	}
+
+	db, err := imapsql.New(testDB, testDSN, &imapsql.FSStore{Root: testFsstore}, imapsql.Opts{
+		LazyUpdatesInit: true,
+		CompressAlgo:    compAlgo,
+	})
+	if err != nil {
+		tb.Fatal(err)
+	}
+	return &Storage{
+		Back: db,
+	}
+}
+
+func BenchmarkStorage_Delivery(b *testing.B) {
+	randomKey := "rcpt-" + strconv.FormatUint(rand.New(rand.NewSource(time.Now().UnixNano())).Uint64(), 10)
+
+	be := createTestDB(b, "")
+	if u, err := be.GetOrCreateUser(randomKey); err != nil {
+		b.Fatal(err)
+	} else {
+		if err := u.Logout(); err != nil {
+			b.Fatal(err)
+		}
+	}
+
+	testutils.BenchDelivery(b, be, "sender@example.org", []string{randomKey + "@example.org"})
+}
+
+func BenchmarkStorage_DeliveryLZ4(b *testing.B) {
+	randomKey := "rcpt-" + strconv.FormatUint(rand.New(rand.NewSource(time.Now().UnixNano())).Uint64(), 10)
+
+	be := createTestDB(b, "lz4")
+	if u, err := be.GetOrCreateUser(randomKey); err != nil {
+		b.Fatal(err)
+	} else {
+		if err := u.Logout(); err != nil {
+			b.Fatal(err)
+		}
+	}
+
+	testutils.BenchDelivery(b, be, "sender@example.org", []string{randomKey + "@example.org"})
+}
+
+func BenchmarkStorage_DeliveryZstd(b *testing.B) {
+	randomKey := "rcpt-" + strconv.FormatUint(rand.New(rand.NewSource(time.Now().UnixNano())).Uint64(), 10)
+
+	be := createTestDB(b, "zstd")
+	if u, err := be.GetOrCreateUser(randomKey); err != nil {
+		b.Fatal(err)
+	} else {
+		if err := u.Logout(); err != nil {
+			b.Fatal(err)
+		}
+	}
+
+	testutils.BenchDelivery(b, be, "sender@example.org", []string{randomKey + "@example.org"})
+}