TP: pool of hashmaps for message bundling

We currently have 2 message bundlers i TP: the default bundler which adds messages to a hashmap and, when full, has the sending thread serialize all queued messages into one and send it, and the transfer queue bundler, which has the sending thread place a message into a blocking queue and the dequeuer thread collect and send the bundled messages.
The disadvantage of the default bundler is that everyone has to wait adding messages to the hashmap, until message bundling has completed. OTOH, the transfer queue bundler blocks all sending threads when the blocking queue is full.

Let's create a third bundler, which behaves like the default bundler, but uses a pool of hashmaps rather than one. This way, when one hashmap is full, the other sending threads don't have to wait until bundling is complete, but can continue adding their messages to a different hashmap.