Mount the SD card asynchronously when it becomes available

Previously we were doing it synchronously, which led to some odd looking
livelock stacktraces... we still don't have a consistent repro, but this
at least makes the stack when we mount a lot more predictable

src/tangara/system_fsm/running.cpp

@@ -40,7 +40,7 @@ void Running::entry() {
     sUnmountTimer = xTimerCreate("unmount_timeout", kTicksBeforeUnmount, false,
                                  NULL, timer_callback);
   }
-  mountStorage();
+  events::System().Dispatch(internal::Mount{});
 }
 
 void Running::exit() {
@@ -72,7 +72,8 @@ void Running::react(const SdDetectChanged& ev) {
   }
 
   if (ev.has_sd_card && !sStorage) {
-    mountStorage();
+    events::System().Dispatch(internal::Mount{});
+    return;
   }
 
   // Don't automatically unmount, since this event seems to occasionally happen
@@ -120,7 +121,7 @@ void Running::react(const SamdUsbMscChanged& ev) {
     gpios.WriteSync(drivers::IGpios::Pin::kSdPowerEnable, 0);
 
     // Now it's ready for us.
-    mountStorage();
+    events::System().Dispatch(internal::Mount{});
   }
 }
 
@@ -145,7 +146,7 @@ auto Running::updateSdState(drivers::SdState state) -> void {
   events::System().Dispatch(SdStateChanged{});
 }
 
-auto Running::mountStorage() -> void {
+void Running::react(const internal::Mount&) {
   // Only mount our storage if we know it's not currently in use by the SAMD.
   if (sServices->samd().UsbMassStorage()) {
     updateSdState(drivers::SdState::kNotMounted);

src/tangara/system_fsm/system_events.hpp

@@ -82,6 +82,8 @@ struct SamdInterrupt : tinyfsm::Event {};
 struct IdleTimeout : tinyfsm::Event {};
 struct UnmountTimeout : tinyfsm::Event {};
 
+struct Mount : tinyfsm::Event {};
+
 }  // namespace internal
 
 }  // namespace system_fsm

src/tangara/system_fsm/system_fsm.hpp

@@ -63,6 +63,7 @@ class SystemState : public tinyfsm::Fsm<SystemState> {
   virtual void react(const audio::PlaybackUpdate&) {}
   virtual void react(const internal::IdleTimeout&) {}
   virtual void react(const internal::UnmountTimeout&) {}
+  virtual void react(const internal::Mount&) {}
 
  protected:
   auto IdleCondition() -> bool;
@@ -101,16 +102,17 @@ class Running : public SystemState {
   void react(const audio::PlaybackUpdate&) override;
   void react(const database::event::UpdateFinished&) override;
   void react(const SamdUsbMscChanged&) override;
-  void react(const internal::UnmountTimeout&) override;
   void react(const StorageError&) override;
 
+  void react(const internal::UnmountTimeout&) override;
+  void react(const internal::Mount&) override;
+
   using SystemState::react;
 
  private:
   auto checkIdle() -> void;
 
   auto updateSdState(drivers::SdState) -> void;
-  auto mountStorage() -> void;
   auto unmountStorage() -> void;
 
   bool storage_mounted_;