Implement timestamp generator configuration

[Lorak-mmk]

Function list:

• cass_timestamp_gen_server_side_new
• cass_timestamp_gen_monotonic_new
• cass_timestamp_gen_monotonic_new_with_settings
• cass_timestamp_gen_free
• cass_cluster_set_timestamp_gen

[muzarski]

This needs to be implemented in rust-driver: scylladb/scylla-rust-driver#262.

[muzarski]

I think we already have everything we need after scylladb/scylla-rust-driver#1128 was merged.