Allow Polling Multiple TaskRuns in One PollTaskRequest

[coltmcnealy-lh]

@eduwercamacaro did fantastic work on performance testing in LittleHorse. He found out that throughput was bottlenecked by the round trip time of the following cycle in the Task Worker:

• PollTaskRequest is sent by the client and received by the server.
• Server sends a TaskClaimEvent to Kafka, then reads it back, and processes it
• Server returns ScheduledTask to the client.

Right now, only one TaskRun can be in that loop at a time for a single worker thread (PollThread in Java, and connection to the LH Server in Python and Go).

We can improve task worker throughput by parallelizing this. One way is thorugh #1084 which allows multiple loops to go on at once.

Another way is to allow one processing loop (i.e. one PollTaskRequest) to retrieve multiple ScheduledTasks.

Both ways are useful, especially when combined together. The specific benefit of this ticket is that it will reduce the number of network requests and traffic, thereby reducing the load on the server.

We can even take it a step further and allow the TaskClaimEvent internal command to support claiming multiple TaskAttempts at one time, which would further reduce the load we send to Kafka.