graphql: Treat missing variables as null values

Instead of immediately reporting an error, treat missing variables as nulls
and let the rest of the execution logic deal with nulls

graph, graphql: Properly validate GraphQL queries before executing them

Uses `graphql-tools-rs` for the actual validation.

Validation can be turned off by setting `DISABLE_GRAPHQL_VALIDATIONS=true` in the environment.

chain head listener: could log be reordered?

chain head listener: add blocking thread pool contention check

chain head listener: more logs

chain head listener: change watcher deadlock detection to avoid spuri…

…ous deadlock detection

chain head listener: change watcher deadlock detection to avoid spuri…

…ous deadlock detection

chain head listener: add number to logging contexts

chain head listener: switch atomic for mutex

chain head listener: moar moar logs