Do not execute process_saved_queries if nothing in semantic manifest has changed

[gshank]

Description

Currently we run process_saved_queries for every saved query in a manifest, even if nothing has changed in the semantic manifest. In the right circumstances this can be a performance problem.

Acceptance Criteria

Check for actual changes to the semantic manifest before executing process_saved_queries for every saved query.

[gshank]

Saved queries, metrics and semantic models are in the semantic manifest. In some cases semantic_models may be "scheduled for parsing" because a model that it refers to has changed, but nothing in the actual definition has changed. If we want to minimize the cases where we run process_saved_queries, it might make sense to create a flag in the partial parser ("semantic_manifest_changed") and set it if we encounter a saved query, metric or semantic model in "handle_schema_file_changes".