[dynamodb] Add HASH_AND_RANGE primary key type

Extend the dynamoDB client to support the HASH_AND_RANGE primary key
type. The default primary key type remains exactly the same as before:
HASH type. And the default behavior and processing logic also remains
exactly the same as before.

What's added is the support to benchmark a primary key type called
HASH_AND_RANGE which is one of two supported primary key types of
DynamoDB. Knowing the performance characteristics of the HASH_AND_RANGE
key is important for some users because that's the only practical way
for them to get strongly consistent query across multiple items.

See documentation in conf/dynamodb.properties for more details.