Use fetch_multi for multiple cache blocks

[sobrinho]

This will improve the performance since we are going to hit the backend
just once to read all keys, as long the cache adapter implements fetch
multi support like dalli.

For example:

json.cache! :x do
  json.x true
end

json.cache! :y do
  json.y true
end

json.cache! :z do
  json.z true
end

This example was hitting the memcached 6 times on cache miss:

1. read x
2. write x
3. read y
4. write y
5. read z
6. write z

And 3 times on cache hit:

1. read x
2. read y
3. read z

After this change, 4 times on cache miss:

1. read multi x,y,z
2. write x
3. write y
4. write z

And 1 time on cache hit:

1. read multi x,y,z

Note that in the case of different options, one read multi will be made
per each options, i.e.:

json.cache! :x do
  json.x true
end

json.cache! :y do
  json.y true
end

json.cache! :z, expires_in: 10.minutes do
  json.z true
end

json.cache! :w, expires_in: 10.minutes do
  json.w true
end

In the case of cache miss:

1. read multi x,y
2. write x
3. write y
4. read multi z,w
5. write z
6. write w

In the case of cache hit:

1. read multi x,y
2. read multi z,w

That's because Rails.cache.fetch_multi signature is limited to use the
same options for all given keys.

And for last, nested cache calls are allowed and will follow recursively
to accomplish the same behavior, i.e.:

json.cache! :x do
  json.x true

  json.cache! :y do
    json.y true
  end

  json.cache! :z do
    json.z true
  end
end

json.cache! :w do
  json.w true
end

In the case of cache miss:

1. read multi x,w
2. read multi y,z
3. write y
4. write z
5. write x
6. write w

In the case of cache hit:

1. read multi x,w

The same rule of options will be applied, if you have different options,
one hit per options.

This is the result of an investigation in application that was spending
15% of the time by hitting the memcached multiple times.

We were able to reduce the memcached time to 1% of the request by using
this algorithm.

Thanks to @samflores for helping me on the initial idea.

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @rwz (or someone else) soon.

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[sobrinho]

We lost the instrumentation with that change but I think we should focus on instrumentation at rails adapters itself instead of Jbuilder.

Make sense?

[sobrinho]

Also, not sure why the build failed, it's working locally and I don't have permissions to rerun the build.

[sobrinho]

This is somehow a new incarnation of #298.