In the Redis driver, only read the msg body when needed

For non-trivial messages, reading the msg body when it isn't necessary
for the business logic can have significant performance implications.

As part of this work, some of the Redis queries were able to be DRYed.

This patch was tested using ZAQAR_TEST_REDIS=1.

Change-Id: I2256404ca9320a22e574625edb229979c1800b40
Merge "In the Redis driver, only read the msg body when needed"