Implement live read replication

This commit adds an http server and client for streaming snapshots
and WAL pages from an upstream Litestream primary to a read-only
replica.

db_bsd.go

@@ -0,0 +1,21 @@
+//go:build !linux
+
+package litestream
+
+import (
+	"io"
+	"os"
+)
+
+// WithFile executes fn with a file handle for the main database file.
+// On Linux, this is a unique file handle for each call. On non-Linux
+// systems, the file handle is shared because of lock semantics.
+func (db *DB) WithFile(fn func(f *os.File) error) error {
+	db.mu.Lock()
+	defer db.mu.Unlock()
+
+	if _, err := db.f.Seek(0, io.SeekStart); err != nil {
+		return err
+	}
+	return fn(db.f)
+}