Convert KJ filesystem API to be entirely threadsafe.

The disk-backed implementations were already inherently threadsafe. Making this explicit allows callers to rely on it.

This is particularly useful for SchemaParser, which is intended to be threadsafe.

c++/src/kj/filesystem-test.c++

@@ -264,13 +264,13 @@ public:
     time += 1 * SECONDS;
   }
 
-  Date now() override { return time; }
+  Date now() const override { return time; }
 
-  void expectChanged(FsNode& file) {
+  void expectChanged(const FsNode& file) {
     KJ_EXPECT(file.stat().lastModified == time);
     time += 1 * SECONDS;
   }
-  void expectUnchanged(FsNode& file) {
+  void expectUnchanged(const FsNode& file) {
     KJ_EXPECT(file.stat().lastModified != time);
   }
 

c++/src/kj/time.c++

@@ -26,12 +26,12 @@
 
 namespace kj {
 
-Clock& nullClock() {
+const Clock& nullClock() {
   class NullClock final: public Clock {
   public:
-    Date now() override { return UNIX_EPOCH; }
+    Date now() const override { return UNIX_EPOCH; }
   };
-  static NullClock NULL_CLOCK;
+  static KJ_CONSTEXPR(const) NullClock NULL_CLOCK;
   return NULL_CLOCK;
 }
 

c++/src/kj/time.h

@@ -63,10 +63,10 @@ constexpr Date UNIX_EPOCH = origin<Date>();
 class Clock {
   // Interface to read the current date and time.
 public:
-  virtual Date now() = 0;
+  virtual Date now() const = 0;
 };
 
-Clock& nullClock();
+const Clock& nullClock();
 // A clock which always returns UNIX_EPOCH as the current time. Useful when you don't care about
 // time.