retry_policy: replace all usages of Box<dyn RetryPolicy> with Arc<...>

[muzarski]

This way, when user wants to reuse a retry policy in multiple places, there is no need for an additional allocation for Box (RetryPolicy::clone_boxed).

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• [ ] I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• [ ] I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• [ ] I added appropriate Fixes: annotations to PR description.

[github-actions]

cargo semver-checks detected some API incompatibilities in this PR.
Checked commit: c958c9b

See the following report for details:

cargo semver-checks output

./scripts/semver-checks.sh --baseline-rev aef0cf5c8dea75d878f0ae8c687213fd9fa287d7
+ cargo semver-checks -p scylla -p scylla-cql --baseline-rev aef0cf5c8dea75d878f0ae8c687213fd9fa287d7
     Cloning aef0cf5c8dea75d878f0ae8c687213fd9fa287d7
     Parsing scylla v0.14.0 (current)
      Parsed [  20.439s] (current)
     Parsing scylla v0.14.0 (baseline)
      Parsed [  19.951s] (baseline)
    Checking scylla v0.14.0 -> v0.14.0 (no change)
     Checked [   0.107s] 94 checks: 93 pass, 1 fail, 0 warn, 0 skip

--- failure trait_method_missing: pub trait method removed or renamed ---

Description:
A trait method is no longer callable, and may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#major-any-change-to-trait-item-signatures
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.36.0/src/lints/trait_method_missing.ron

Failed in:
  method clone_boxed of trait RetryPolicy, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-aef0cf5c8dea75d878f0ae8c687213fd9fa287d7/28587ea95124515726f41c658488682589e689eb/scylla/src/transport/retry_policy.rs:34
  method clone_boxed of trait RetryPolicy, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-aef0cf5c8dea75d878f0ae8c687213fd9fa287d7/28587ea95124515726f41c658488682589e689eb/scylla/src/transport/retry_policy.rs:34

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [  40.550s] scylla
     Parsing scylla-cql v0.3.0 (current)
      Parsed [   9.944s] (current)
     Parsing scylla-cql v0.3.0 (baseline)
      Parsed [  10.046s] (baseline)
    Checking scylla-cql v0.3.0 -> v0.3.0 (no change)
     Checked [   0.106s] 94 checks: 94 pass, 0 skip
     Summary no semver update required
    Finished [  20.143s] scylla-cql
make: *** [Makefile:61: semver-rev] Error 1

[wprzytula]

We do lose something. Namely, Box has an optimisation which makes it not allocate anything for ZSTs, and Arc allocates always (its control block). However, I don't know if the optimisation is viable for Box<dyn Trait> with ZST inside.