multi: boostrap local cache

[Kiko Beats]

Normally you are going to combine keyv/multi remote strategy with a local in-memory strategy.

Right now, when the value is not on the local store but it's present on the remote store, it's copied:

https://github.com/microlinkhq/keyv/blob/master/packages/multi/src/index.js#L36

What could be cool there is to add a way to bootstrap the local strategy by copying the content from remote.

Something like:

const keyv = new Keyv({
  store: new KeyvMulti({
    local: new Map(),
    remote: new KeyvRedis(),
    bootstrap: async () => {
      const keys = await this.remote.redis.keys('dns:*')
      return this.remote.redis.gmet(keys)
    }
  })
})

Where the bootstrap code is run at the beginning to bootstrap the local storage.