refactor updatePoolWeights to avoid on-chain calls

[mendesfabio]

On every Swap event we update pool's tokens weights if they're variable (e.g. LBPs) by making on-chain calls. We could improve this and avoid the calls by calculating the weights at the timestamp of the swap using the GradualWeightUpdate entities. This is pretty much a refactor of the updatePoolWeights function.

Please use the comments to document your journey from cloning the repository and setting up the environment to coding the solution. You can ignore most of the README and it has instructions to run graph-node locally and that's not what we're looking for.

https://github.com/balancer/balancer-subgraph-v2

Relevant commands are the following:

1. yarn
2. yarn generate-assets
3. yarn codegen
4. yarn deploy

* Recommend yarn since that's the package manager we're using and there's already a yarn.lock file.

[thetroyharris]

• Repo cloned
• yarn installed, env setup, dependencies installed
• yarn generate-assets no issues
• yarn codegen no issues
• Deployed a mainnet copy of the current subgraph to my own endpoint

Will begin working on refactoring the function.

[thetroyharris]

Draft pull request has been opened. See #541
I have deployed to my own endpoint and will begin testing the results of these changes.