refactor(processor): improve batch processor shutdown and channel handling

[mattevans]

In an attempt to resolve nil items being run through the batch processor...

WARN[0250] Attempted to build a batch with a nil item. This item has been dropped. This probably shouldn't happen and is likely a bug.  node_record="enode://xx@xx:xx"

... this PR:

• Refactors channel closure in the batch builder to handle shutdown gracefully
• Improve queue draining sequence to prevent race conditions during shutdown
• Wait for in-flight batches to complete before closing channels