Request execution API changes - umbrella issue

[wprzytula]

This is an umbrella issue to group all changes regarding the request execution API, as there are a number of them planned before the API gets stable.

Tasks

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1. Replace Query with a more fitting name #713 - we're currently planning to implement the Executable trait
2. Move set_timestamp out of Query/PreparedStatement #262 - necessarily consider this when implementing Executable
3. Internal reusable CQL statements should be prepared #417 - this needs to be resolved if we decide to forbid executing unprepared requests with values (which we want to, in order to make requests type-safe)
4. query_paged is a misleading name #940 - covered by Executable
5. Introduce BoundStatement #941 - solves multiple issues, e.g. Enhancement Proposal: Simplifying Token Calculation for High-Frequency Append-Log Style Operations #974
6. Provide an option to bind values to each separate statement in a batch #108 - possibly solved by Introduce BoundStatement #941
7. Avoid allocations in CachingSession::execute #475 - need to take such things into consideration when designing new API and Executable trait.