v0.20.0

• [Breaking] Deprecate and remove #each_batch due to data consistency concerns.
• [Enhancement] Bump librdkafka to 2.6.1
• [Enhancement] Expose rd_kafka_global_init to mitigate macos forking issues.
• [Enhancement] Avoid clobbering LDFLAGS and CPPFLAGS if in a nix prepared environment (secobarbital).
• [Patch] Retire no longer needed cooperative-sticky patch.