c-s user: Implement runtime for user command

[muzarski]

Fix: https://github.com/scylladb/cql-stress/issues/79

This PR introduces a simple runtime for user command.

One operation consists of:

• sampling a pair (row, statement to execute). For now, the statements (provided by the user via queries parameter in yaml profile) are sampled in a round-robin fashion but this will be randomized in the future. See the comment in the code next to OperationSampler. Related issue: https://github.com/scylladb/cql-stress/issues/80
• Execute a sampled statement with variables bound from the sampled rows. If the operation fails, it will be retried with the same statement and row. This is why both of these are cached in case of failure.

Tests

This PR introduces a test case that checks compatibility with original c-s for the blob CQL type. To see the tests for other types, see this PR https://github.com/scylladb/cql-stress/pull/83.

[muzarski]

v2: addressed review comments, and replied to some of them