Make claim batch size configurable in benchmark tool

This patch adds a new config option so that the user can control the
number of messages the consumer claims at a time. The default is
5 rather than 1, since claims are designed for batch processing
of tasks.

Change-Id: I18d58d09526cf0f7121a1b3ef532a4f15ff206e7
Merge "Make claim batch size configurable in benchmark tool"