Improvement/record count refactoring

[tiagopog]

Closes #78

What's this PR do?

It brings some refactoring + fixes to the record count feature.

TODOs

• Fix the execution of slow count queries when using AR objects with eager load (i.e. count with left joins);
  • If for some reason the effectiveness of the query is affected for skipping the left joins (e.g. where clauses) then rollback to the slow count;
• Raise an error if no count strategy can be applied;
• Apply memoization on the record count;
• Write specs;
• Bump up the gem's version.

Background context

At Beauty Date we have a production HTTP API fully based on JSON API using jsonapi-utils to parse request e build responses. While optimizing an endpoint that was suffering from "N+1 queries" problem, one of our developers (@giovannibonetti) noted that when using eager load on large tables it actually ends up with an extremely low count query because of the left join associations.

Manual test

# Explicit count
jsonapi_render json: User.all, options: { count: 42 }

# Array count
jsonapi_render json: User.all.to_a

# ActiveRecord count
jsonapi_render json: User.all

# ActiveRecord count with eager load
jsonapi_render json: User.all.includes(:posts)

# ActiveRecord count with eager load and where clause
jsonapi_render json: User.all.includes(:posts).where(posts: { id: Post.first.id }) 

[tiagopog]

@giovannibonetti I still need to write some unit tests but since it's +-90% done I think you can already have a 👀 on this PR if you want.