Improve `mqbs_datastore.t.cpp` unit tests

[678098]

Is there an existing issue for this?

• I have searched the existing issues

Current Behavior

The current implementation is mostly a copy-paste of bmqt::MessageGUID unit tests.
It has several flaws:

1. These comments are misleading and not true:
   

   blazingmq/src/groups/mqb/mqbs/mqbs_datastore.t.cpp

   Lines 175 to 177 in 536a3be

   	// Above value is just chosen after looking at the number of collisions
   	// by running this test case manually. In most runs, number of
   	// collisions was in the range of [0, 3].

2. The way how mqbs::DataStoreRecordKeys are generated is not representative to the usage pattern in the broker:
   

   blazingmq/src/groups/mqb/mqbs/mqbs_datastore.t.cpp

   Line 249 in 536a3be

   mqbs::DataStoreRecordKey key(i, 7);

To get it right, both args should be modified.

Expected Behavior

No response

Steps To Reproduce

BlazingMQ Version

all

Anything else?

No response