Allow to disable GC within documents using env variable/CLI

[krassowski]

References

• closes Allow to run with garbage collection disabled? #421

Description

Adds an option to disable GC within documents (distinct from GC of documents in multi-document mode) via:

• CLI by passing --skip-gc to either of y-sweet serve or y-sweet serve-doc
• Y_SWEET_SKIP_GC environment variable

I was not sure if I should also add the environment variable to y-sweet-worker for feature parity but given that it is deprecated I went for a smaller changeset and only added it in the server.

I wonder if the configuration for toggling GC of documents themselves (the doc_gc option which is not exposed by clap) would be advisable, but I think it is already possible to configure checkpoint_freq_seconds instead which can be set to a very high value that is probably better than turning it off altogether.