Remove connect, disconnect for session strategies

[emmatown]

This pull request removes connect and disconnect from the SessionStrategy interface, and replaces the session-store-redis package with an example.

We shouldn't have created a package for what can be effectively reproduced with:

const sessionStrategy = storedSessions({
  store: ({ maxAge }) => ({
    async get(key) {
      let result = await redis.get(key)
      if (typeof result === 'string') {
        return JSON.parse(result)
      }
    },
    async set(key, value) {
      await redis.setEx(key, maxAge, JSON.stringify(value))
    },
    async delete(key) {
      await redis.del(key)
    },
  }),
  // ...
})

export default withAuth(
  config({
    db: {
      // ...
      async onConnect() {
        await redis.connect()
      }
    },
    // ...
  })
)

And an redis.connect() in the db.onConnect hook.

Unfortunately db.onConnect probably should have been called onStartup[but before HTTP].

At the moment, db.onConnect happens to be sequentially when we connect the Prisma client, but strictly has no relationship to if and when the Prisma client is connected or disconnected.
This can lead to DX confusion when you use a context directly from getContext.

[dcousens]

LGTM