[C++] Simplify concurrency in as-of-join node

[rtpsw]

Describe the enhancement requested

Currently, as-of-join node has internal invocations that require thread-safe synchronization but can be simplified to not require this:

• Invalidation of the key hasher.
• Updating of the memo-store's time upon receiving a batch.

Both of these can be moved to the processing thread, instead of invoked from the input-receiving thread.

Component(s)

C++

[rtpsw]

take