Detect changes in eventTrigger definitions

[samtstern]

Description

Fixes #2533

Scenarios Tested

1. Create a function exports.firestoreFn = functions.firestore.document('/test/{doc}').onWrite(...)
2. Write a doc in /test the emulator UI, see that the function executes
3. Change the trigger to be functions.firestore.document('/test2/{doc}')
4. Write a doc in /test, nothing happens.
5. Write a doc in /test2, see exactly one execution.

Logs:

$ firebase --project=demo-project-12345  emulators:start
i  emulators: Starting emulators: functions, firestore
i  emulators: Detected demo project ID "demo-project-12345", emulated services will use a demo configuration and attempts to access non-emulated services for this project will fail.
i  firestore: Firestore Emulator logging to firestore-debug.log
i  ui: Emulator UI logging to ui-debug.log
i  functions: Watching "/private/var/folders/xl/6lkrzp7j07581mw8_4dlt3b000643s/T/tmp.bH599aKP/functions" for Cloud Functions...
✔  functions[us-central1-firestoreFn]: firestore function initialized.

┌─────────────────────────────────────────────────────────────┐
│ ✔  All emulators ready! It is now safe to connect your app. │
│ i  View Emulator UI at http://localhost:4000                │
└─────────────────────────────────────────────────────────────┘

┌───────────┬────────────────┬─────────────────────────────────┐
│ Emulator  │ Host:Port      │ View in Emulator UI             │
├───────────┼────────────────┼─────────────────────────────────┤
│ Functions │ localhost:5001 │ http://localhost:4000/functions │
├───────────┼────────────────┼─────────────────────────────────┤
│ Firestore │ localhost:8080 │ http://localhost:4000/firestore │
└───────────┴────────────────┴─────────────────────────────────┘
  Emulator Hub running at localhost:4400
  Other reserved ports: 4500

Issues? Report them at https://github.com/firebase/firebase-tools/issues and attach the *-debug.log files.
 
i  functions: Beginning execution of "us-central1-firestoreFn"
>  test/qJN4KLuFOJItu0iIUjcK
i  functions: Finished "us-central1-firestoreFn" in ~1s
✔  functions[us-central1-firestoreFn]: firestore function initialized.
i  functions: Beginning execution of "us-central1-firestoreFn"
>  test2/6FyUaaYWMaILYtXwb3vK
i  functions: Finished "us-central1-firestoreFn" in ~1s

Sample Commands

N/A

[yuchenshi]

Any chance we can get an e2e test?

[samtstern]

@yuchenshi getting a meaningful and reliable e2e test would not be possible in our current test setup because it would require modifying functions source files to hit the file watcher.

[yuchenshi]

All right, let's go for it without automated tests then.